You might be able to try using 3D printing resin if you just want to seal and harden the phone prior to painting. So far it hasn't eaten through anything I've used it on (though I've only tried it on closed cell foam, not open cell). Brush it on thin in patches and hit with UV light for 2-3 seconds. Move on to another patch until it's all covered. Go back over it with 2-3 more coats to give it more rigidity and should be good. A 500ml bottle will last you pretty long due to the layers being so thin. I'd say this shield would only take 1/6 of that bottle if that. If you go with Elegoo's ABS-like resin, it's easily sandable too.
