What kind of tools do you have available? That's something to consider when deciding on materials.
Metal is good / strong.
Plexiglas is nice to finish and can be clear, but it is expensive and not very stiff.
Plywood in inexpensive and strong. And may be easier to work with.
Fiberglass / resin gives you the ability to make complex shapes.
Or, you could combine materials - make a metal bracket to attach to the motor then bolt it to a plywood deck.