Don't use bands:
Whatever you do, I suggest you don't use any bands. If your body mechanics work a certain way, you need to consciously work on it if you want to change it. Artificially restricting your legs to move a certain way is likely to cause injuries and unnecessary strain on the joints. When you shadowbox, go hard and fast so you get tired, and then keep shadowboxing while only focusing on your stance and legs.
Sometimes, while boxing, after some part of round passes, I find myself in a wide stance - with legs wider than shoulders.
You are asking the wrong question. Before asking "How can I fix the problem?", you need to ask "Why am I doing this all the time in boxing sparring, and not when I am doing bag work or pad work?"
To cure a disease, you need to find its source; you need to understand what is causing it, right?
There are at least two reasons why you might be going into a wider stance during your boxing sparring.
After a few rounds, you are fatigued. Your technique is no longer giving you the results you want. Your hands are worn out, and so are your legs. Your punches don't have the "snap" anymore. You are keeping your punches tight and clean, but you feel they are no longer doing damage. They don't have the speed to make that damage.
So. what do you do to get the same strength or power behind your punches that you had in the first two rounds (when your punches were snappy and clean)?
- You compensate. You widen your stance and draw the force from the ground. In simple words, you are loading up; widening your stance adds power to your shots.
Why you don't see this bad habit when you do your pad or bag work?
- That is because the pad holder adjusts the pads for you — you are always able to hit the target, and the noise feedback tells you your punches are landing (you don't need power to hear those snaps with the pads). When you are sparring, the clean techniques in the later rounds (when you are fatigued) may not elicit any reaction from your sparring partner. This may make you feel that you need to hit that dude harder, harder, harder. With the pads and the heavy bag, you don't need that, the sound feedback is readily available.
In the first two rounds, you are moving like a butterfly, using that great footwork of yours! But in the later rounds, your legs are worn out. You are not able to move around that swiftly anymore. And once your opponent starts throwing punches in bunches and you can't get out of that exchange, your body natural reacts by widening your stance. If you are being hit by a barrage of punches (or even a few strong ones) your natural tendency would be to widen your stance to brace against those hits. Your body does that so that you don't fall over.
Another thing that happens when we are getting hit by combos is that we tend to close our eyes, and when that happens, we often widen our stance and plant our feet firmly to the ground. What we should do is parry and roll with the punches, and not "shell up".
Remember that at this point you are not able to move as swiftly as you were able to in the first two rounds. May be (1) your legs are not conditioned enough, (2) you are gassed, (3) your muscles are too tight and tensed, (4) you are not breathing, (5) you are feeling dizzzzzy and having difficulty maintaining balance, etc.
Why you don't see this bad habit when you do your pad or bag work?
- Because they don't hit back! And so your body does not need to brace against any thing.
I presume that a combination of these factors (both offense and defense) and others that I have not considered and don't know are what is causing you to go into that wide stance a few rounds in to the session.
So why am I not doing this wide stance in my kickboxing sessions?
- That is because in kickboxing (KB) or Muay Thai (MT), we are specifically paying attention to our lower body which in boxing we are not. We are concerned about blocking low kicks with our shins, we are concerned about devastating inside low kicks if we were to get into a wide stance, we are concerned our opponent might get into a better clinch position if we are too wide and low. Our minds are more active in KB/MT (there are more sources and varieties of attacks - punches, kicks, knees, elbows, clinch). We are paying attention to all our limbs. Then there is the offense part. We want to be able to throw those round house kicks, teeps, spinning side sides, knees, etc. We can't do any of those from a wide stance. Because we are so much concerned about and focused on our lower body in KB/MT, it keeps us from getting into poor, wide stances.
So, how do I fix it?
I don't know. I am not an expert or a coach. This happens to me as well. When I am in the 3rd round, I am being a warrior and I want to hit my opponent hard. In the first two rounds I am having fun focusing on footwork and movement. Everything I mentioned applies to me as well. Overtime, I was able to work on the problem.
Here is what worked for me. Bend more at your knees, instead of widening your stance too much. The extra bent knees allows you to quickly spring inside the pocket (i.e., close the distance and get in range to unleash havoc on the body) with the gazelle punch or the long lead uppercut. It helps you to spring out and escape when your opponent starts a combo. Basically, bending your knees a bit more from time to time helps with quick bursts. And you are able to move more swiftly when your opponent throws a barrage of punches (contrary to how we freeze with a wider stance when being hit by punches). So, bend your knees to stay low instead of widening your stance to stay low.
I learnt the hard way in KB. I was testing for a third grade belt, and I had to spar with one of the black belts (they are the 8th grade belts) in my gym. This was after a grueling session of 2.5 hours of combos and partner drills, pad work, kick shields, and 7 rounds of serious sparring with other test takers. The black belts are judges and they come in fresh to spar with those testing for third grade and above. I was dead tired and I resorted to widening my stance as he unleashed a fury of punches that I could barely see. I was dizzy and my head was spinning. I was gassed out. When I went back to the gym, I asked my mentor what I could have done to not get stuck in that compromised position in the corner. He said never try to brace against punches with a wider stance - you might think that the combo will end and you will soon be able to throw yours and get out - but you won't. Once you get into that wider stance while being hit by combo after combo, a great fighter will finish you off. So when you feel you are going to get stuck, raise your guard and move the hell out.