Cut a severe taper (70 degrees) using a compound mitre saw
Cut it perpendicularly.

70 degrees from 90 degrees (ie: a right angle) is 20 degrees.

Square the end of your taper and put a mark in the middle.

Set your saw on 20 degrees and place the mark so that the taper makes a T with the cutting rail, cut one side, flip the taper over, cut the other.

If you're concerned about stability - make an L shaped jig which you can put up against the rail and hold the taper against to keep it straight.