I'm not precisely qualified to answer this, but perhaps Janet Rosen's description of how to add modesty straps to a dogi will help? Ms. Rosen provides preliminary diagrams and advice here.

This doesn't address the "slim" issue, but it does address what I'm reluctant to call the "modesty" issue.

I'm not precisely qualified to answer this, but perhaps Janet Rosen's description from an AikiWeb forum thread of adding modesty ties will help.

If all you need is modesty ties, buy a long shoelace, cut it in half, sew the halves where you want them. Snap tape works as well.

Janet Rosen has also written a column on AikiWeb about customizing the dogi which contains diagrams as well.

I have experimented with a variety of closures, including snap tape and velcro, and am happiest with a pair of white or natural (to match the dogi fabric) sneaker shoelaces. One lace cut in half will secure the waist, the other cut in half will secure the lapel. Put on the gi top, over the gi pants and belted as usual, and use straight pins to locate where you want the ties to be. For the waist ties, the left side tie will go on the inside of the top (wrong side) and the right side tie will go on the outside of the top (right side). For the upper lapel ties, the right side tie again will go on the outside of the top (right side), but it is easiest to secure the ties if the left side tie is directly at the edge of the gi top.

This doesn't address the "slim" issue, but it does address what I'm reluctant to call the "modesty" issue.

I'm not precisely qualified to answer this, but perhaps Janet Rosen's comment in this thread will help? I believe this expands on the idea.

I'm not precisely qualified to answer this, but perhaps Janet Rosen's description of how to add modesty straps to a dogi will help?   Ms. Rosen provides preliminary diagrams and advice here.

This doesn't address the "slim" issue, but it does address what I'm reluctant to call the "modesty" issue.