Mega Machine's Megalithic uppers are one of the better uppers for mounting a rail mount m203 to. They've been discontinued and JSE is unloading their remaining stock. For $487 you get the upper and a free Wilson bbl. Damn good deal...

Linky

This is the upper I use for my 203. Just thought I'd pass it along. These days AR rails that are good to go for 203s are few and far between, unless you want to try using an mlok/keymod rail with pic rails attached...