[DonRotolo]

There are many grades of plywood; you want aircraft plywood if possible. It uses many layers of light wood.

regular plywood, like used for house construction, uses only a few layers, and is not as strong (or light) as aircraft plywood. Marine plywood uses a few more layers, but isn't as light. You might consider making your own plywood, two 1/4" aircraft ply outer layers with foam and 1/4" spacers in-between - lighter and just as strong.

For stiffness, use pieces mounted 90 degrees to the plane - glue and screw.

It would be heavier than aluminum, but plenty strong. Use some aluminum U channel along the outer edge to give very good impact strength.

But nice concept!