Marine grade plywood, 5-ply is preferable for weight distribution. PLEASE use a flame retardant material on them after fabrication. This is required per FAR 23, as well as the old CAR-3 that our aircraft were manufactured under. Polyurethane sealers are sure pretty, but NOT flame retardant. Search Amazon.com for 'No-burn original' and give them several applications.
ABS Tech Advisor