A basic mold form for creating a silicone mold of flat things. The photo is 130mm x 170mm x 20mm but you can change the size in Fusion360 using the user parameters (click on the sigma icon). I've also added a few other sizes that may be useful.

It has a gutter to trap overfilling resin. I use carpet tape or spray adhesive to hold parts down on the flat bottom.

I'm using it to cast replica replacement pinball playfield inserts that are no longer available.