We have a few training tools like these at my school where you have to fill the base up with something to weigh it down. One of the items we have is filled with water, but is smaller than the equipment you've described. Water, which has a density of 1 g/cm3, keeps our equipment weighed down enough that the base doesn't move when you hit the target (which is on a spring). The downside is that water can leak out of it if you're not careful with it or if the container wears out, breaks, or is shoddy.
One of the other larger items we have, similar to the item described in your question, is filled with regular playground sand. It holds down our item pretty well and is sufficient enough that even really hard hits will not knock the thing over (even though it may move back a few inches). It does take some heft to push it around the room if you want to move it, but it works well enough for our purposes.
Another option is concrete which has a density of 2.3 g/cm3 (give or take, I've found various references citing different values depending on the type of concrete) which is great than water's density. I haven't had experience with filling these types of equipment with concrete, but I do recall from my childhood that the concrete will expand and warp the shape of its container. My dad filled buckets with it for various purposes (which my 10-year-old self thought was heavy to pick up).
Ideally, you should have some method to measure how forceful the hits will be to the training tool to calculate which material is best based on the density and the weight given the volume. It depends on who is hitting the bag and that person's strength. At our school, we have lots of youths who can't move the bag no matter how hard they hit. Some of the adults though can keep moving the bad slowly over time with enough forceful hits. Moving it back isn't too huge an issue.