My wife as accumulated a collection of card tables over the years. One of them permanently resides just outside my darkroom, and I have placed a sheet of "homosote" on top of it. This gives me a fairly large area with a cheap, sacrificial surface that I can use to cut mat board.
I buy full sheets at the art supply store, bring them home, and then use a mat cutter/box cutter and straight edge to cut them down to the size that I want - usually, 16x20.
The straight edge that I use for this is a Logan straight edge that I picked up at a garage sale years ago. But I have seen straight edges at Home Despot that would work just as well.