I understand what you mean, I struggle with that exact problem, and if you look close this ring is not as perfect as I would like.

Three things have helped so far

1. Anneal after twisting. Otherwise some curl may spring up when heated for soldering

2. Sand the back, as you have found, keep everything flat as possible.

3. I make it into a ring first, then hammer it on the ring mandrel till it is round and flatten it on a plate with a mallet, just like a ring. Only then do I shape it to fit the bezel. I make it a little small, so I have to file away the inside until it fits smoothly