I trimmed the thighs down a lot, and used the scraps to reinforce the posterior piece. It's much stiffer now, in a good way:
Per Tony's feedback, I also cut the thickness of the ab boxes down quite a lot. Annoyingly, the huge globs of milliput I used got in the way of the new shorter boxes, so I had to dremel them down as best I could. This also means I'll probably just end up gluing the boxes on that I had to cut the nylon bolts off of. But I do think they look much better now.