What do you think of harlequin rasboras?
My betta totally ignores them. Their fins are quite small, maybe that's why, who knows. They're a lovely raspberry color, with a striking black triangle on the back half of their body.
Putting in plants-- fake or real-- helps too, I think... out of sight, out of mind, I guess.
Some bettas will tolerate no tankmates whatsoever, so it does depend on the temperament of your particular betta.