Many years ago, before 3D printing existed, I made lens boards (or is it lens 'plate'?) for my Linhof Technika out of black Plexiglass.
This can easily be worked on and shaped the way you want it without machines nor sophisticated tools: a cutting lath, a Staley knife, a strip self-adhesive foam, a bell drill, sanding paper, and some muscle power is what you need.
The thickness of the Plexiglass is of lesser importance as you can easily chamfer the edges to have it fit behind the notches.

Oh, and an original Technika lens plate to copy...