You need to make a chuting/shooting board or whatever the equivalent is for a belt sander. One option would be to clamp the sander down and build a jig that allows you to bring the board up to it at the rquired angle.
Alternatively, most circular saws have the option to angle the blade.
how can i accurately and repeatedly cut/sand/file the edge off of a sheet of ply/mdf at a 69 degrees?
i need a jig to hold it at the correct angle so i can then sand it on a beltsander but cant really visualise how it might look.
poo added for scale