Ideally you want some way to transfer the load to the plywood formers. A hardwood block across the bottom of the plane, best if it was wide enough to span between two formers.

Or, you could make your own plywood formers - two 'U' shapes that go across the bottom and up the sides. Look here starting in post 109. Just use plywood instead http://www.rcuniverse.com/forum/rc-p...l#post11285418