I assume we're talking an ARF here? If so, most ARF's need a bit of additional strength in the firewall area anyway, so I make a thrust wedge from a piece of birch plywood. This goes between the standoffs and the existing firewall and gives a perfectly flat surface for the standoffs to mount to. I usually glue this to the existing firewall and motor box for additional strength.

I usually start out with ply that is 1/4" or so thick and tack glue it to the end of a 2x4 or something similar. Then support the 2x4 at the proper angle and run the ply through a table saw or even a bandsaw. When done, I true up the cut surface with a sanding block, pop the ply off the 2x4 and clean up the second surface. Takes longer to describe the process than to do it. Just one method and I'm sure there are dozens more.