I needed to insulate a house I've been building forever and wanted clean cuts so I didn't have any Voids in my insulation.
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 had some long strips of 3/4 inch plywood ripped six inches wide, but anything handy will do. I made it about 30 inches long so it would do a 23 inch wide piece of insulation without having to worry about centering it.
I used two 3 inch door hinges and some screws and glue.
Step 2: Cut Pieces
From the six inch wide pieces of plywood I cut two lengths 28 inches long, one three inches long and one five inches long.
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
I glued and screwed everything together as shown adding a spacer so when the guide was lowered it allowed about 3/4 of an inch for the insulation to compress and this seemed adequate for anything from R-11 to R-30.
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.