I made this from some scraps of wood and two hinges. It holds the insulation so it can be cut with a box cutter in a single pass.
Step 1: Assemble Materials
I used two 3 inch door hinges and some screws and glue.
Step 2: Cut Pieces
I ripped one of the long peices into two three inch strips and then cut out a notch so they formed a slot when rejoined.
Step 3: Assembly
Trial and error determined that it works best with the paper side down and with a good blade on a box cutter a single pass will slice through and give you crisp corners for a good fit.