The two arch elements clamped, and the first, inner cut, being refined. I used a Sabre saw to make the cuts. Clamping the two "boards" together to cut them, caused the plastic material to melt and refuse behind the cut, though a single cut worked well.
To refine the edge, and smooth out the waviness from the hand cut, I'm using a Sureform rasp, and cabinet makers rasps and files. These are tapered, one side flat, the opposite slightly round. Final finish with a random orbit power sander. The clean edge is then used as a reference to mark the outer cut. Another "tool" used for this are a pair of Wells-Lamont mechanics gloves, going by the name "Grips", as rasps and files tend to work on the hands as well.
The finished arch elements.