Here's a jig I made to help me cut out fabric with a hot knife. It assures me of nice straight edges, and helps hold the fabric in place.
It's made of 1/4" MDF although some plywood work work fine. The bottom is about 7" wide. It has two screws/pegs, one in each end to hold the straight edge. The straight edge is 3" wide and has a notch in each end to accept the peg in the base. The pegs keep the staright edge from slipping while cutting with the hot knife.
The straight edge is 3" wide because I was cutting mesh baffles for a quilt project. When I need a baffle of a different width for a summer or winter quilt, then I will make another guide of the appropriate width for those baffles. Having the straight edge the width of the baffle allows you to line one edge of the mesh up to the edge of the guide and cut on the other side. No measuring errors this way.
Using something made from wood allows the hot knife to retain more of its heat. A metal straight edge will act as a heat sink and cool the knife decreasing its melting ability.
This has been a real benefit when doing projects. Hope it helps someone else.