Making one with a heavy poster board is really pretty easy. Though they might be out of orange this month 🎃
Measure twice, use a square, and fold against a good edge. No free hand!
Measure 1/32-1/64" oversize. You want the foam to have room to go in.
(A manilla folder mock-up for sizing?)
Print a photo of the engine on it
Or your model railroads name & logo?
.....No running with the scissors
A razor and a square/straight edge will give better looking, uniform edges, especially on lengths. (cheat with scissors in the corners )