Finally getting around to building frame for my layout

[Russ Bellinis]

I would use either dimensional lumber or 1x4 ripped from plywood for a perimeter frame, and then support it underneath with the door skin. Foam is extremely stiff and resistant to bending, but it is brittle and will break easily. The perimeter frame will protect the edges from being broken if bumped, and the door skin underneath will protect it from breaking. A bit of molding on the inside of the perimeter frame at the bottom will provide a lip to support the door skin and foam on.