Re: need unusual picatinny rail
I am not sure if anyone makes a 40MOA picatinny rail blank, but it wouldn't be that hard to machine a 0MOA/flat blank with a specific cant/angle as long as you can find a competent machinist with the proper equipment to taper it for you.
Check the following sources for picatinny spec blanks in various lengths:
Evolution Gun Works
Mounting Solutions Plus
Nightforce (makes a 20MOA canted blank)