I bought a WW boat the had the nose poured full of foam instead of footpegs. I'm not sure if it has different composition than the spray foam.
If you have a way to secure the foam you could line the space with a non reactive plastic then fill the plastic with foam making it removable. Something as simple as a couple cords embedded in the foam and going to a deck fitting could work to secure it.
Full Size Sail Rig
Kayak Carrier Kits
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