Landing gear mounting plate should be plywood, 1/4" probably. Put some doublers on the fuselage sides (1/8" ply) to support it. Install 1/4" T-nuts into plywood and use 1/4" nylon screws to mount the gear to the plywood. On a bad landing, all you should do is snap the screws. Keep a few extra screws on hand and you should be able to repair in a few minutes. The plywood should run full width of fuselage and @ 2" front to back, a little more if you have a wide bracket. Balsa just doesn't have the strength for lndg gear mtg, especially on a trainer where you are expecting hard landing and more especially on grass fields