Stucco craftsmen use styrofoam to form up window openings, column
decorations, etc. They cover with fibreglass. I've vote for plywood,
since most of how to bond plywood with glass is known.

I made a set of "Jesus boots" once, big long blocks of styrofoam with
horizontal fins on the bottom surface, retracted flush by water
pressure of forward movement. I used fibreglass to seal all surfaces.
The intent was to be able to walk across canals and waterways of the
Louisiana marsh. The fibreglass made the surface quite tough and
ding-resistant.

I will leave that skill to the Master......... I could never keep the
pair close enough together for very long to keep me balanced topside.

Fibreglass and plywood; that's the ticket!

Denny Hugg