So I'm wondering if anyone has any advise on the best insert to wear with regular RS flats? (Not caged) I have a pair of size 40 denim ones and there is a bit of a gap in the back. I tried the 39.5 but those felt too snug. I'm hoping I can just wear an insert with my them and won't have any slippage issues. Also, I know I have to be careful since they are cut so short around the foot, so that my foot doesn't sit too high with the insert. Anyway, any advise would be truly appreciated!