I needed something to guide my foamboard cuts for TTRPG terrain building, so I made these two stackable jigs in TinkerCAD. One is for cutting squares (shown) and one is for cutting circles. The third item slots into the square jig to align corners

One side of each surface has small spikes, and one side is smooth. The spikes help it stay secure to the surface, but will leave little marks. Use the smooth side if you want to avoid this.

Total dimensions:
Each jig is 1.25" x 1.25" x 0.125" (excluding small spikes. included, they bring the height to ~0.21" depending on printer resolution)
The corner-aligner is 0.625" x 0.625" x 0.5"