I would use 1/8 inch Luan plywood for strength. Old paneling or bead board is ideal. These can be cut with a knife.
What kind of track are you using? Tubular, FastTrack, Gargraves, Atlas? You can use the actual track as yor templates for cutting. Cut the plywood in the longest length possible, attach track to the top and the trestle to the bottom. Make sure that the substrate joints rest on a trestle bent.