I do have two part epoxy and fiberglass but mine is slow set and of course it is expensive. A brace down the middle would be a good idea.
I have had a few experiences with Great Stuff foam. I sprayed it on the bottom of my kayak boat seat for a glove fit but over weeks it kept warping and changing in a shrinking way. I also used it around one of those rubber farm animal drinking pans to give it extra stability while floating in the pond and over time in the water it deteriorated badly. I think it must be open cell. Also I once put it inside a long aluminum sailboat mast. To get it the full length I drilled small holes at intervals along the mast and shot Great Stuff in them. Below is blue board foam that was glued with two part epoxy to build up a large chunk of it.
