Yes the biggest challenge with spray foam, or even better the pourable expanding stuff, besides using in the correct environmental conditions
is the tremendous pressure in all directions as it expands.
Can buckle and warp even pretty thick metal walls, especially if there aren't large-area "escape routes", or even if there are but you try to do too much at once.
With a weak-material "mold container", it helps to do it in very thin layers, sometimes necessary to work with just a fluud ounce at a time.
In warm conditions at the right humidity, the setting time between layers can be well under an hour, but for full curing, reduce the risk of long-term offgassing, best to follow the spec'd instructions for that specific product.
I'm thinking for foamie use gap-filling, corner gluing boards, creating a 3D single cohesive foam structure first, then fairing away any irregular bulges etc, then applying the PMF canvas afterwards.
For in-use repairs, maybe a non-expanding waterproof "bondo" type filler would be better, not necessarily cheap automotive stuff but marine type, microballoons / fumed silica filler in epoxy.