Oh dearie me.
Still, call it a learning opportunity. At least you didn’t set it on fire. There are those that have managed to do that. And next time, you’ll use more resin. Better to waste a little resin than…yeah.
And, a picture or several would help a lot.
In the meantime, and all could change once we see it, I’d suggest you treat it like a ding repair. Open it up, gently remove what I’ll bet is ‘former foam’ gone soft and shrunken, fill and sand to fair it to the shape it should be, glass over.
Now, some people do dings and the irst thing they do is whip out a router and attack, attack, attack. Generally it’s a very bad idea, they make a simple, easy repair into a giant clusterfuc#. This might be that rare exception- if you have a plunge router and you’re good with it you might want to think about that. . Otherwise, no.
What to use for filler? You could slather in resin plus foam dust or cabolsil/aerosil mixed in to thicken the resin to a useful filler paste- but it’s heavy, hard to sand and you wind up with something pretty obviously a repair.
You could very carefully cut a chunk out and glue in a precisely fitted piece of foam ( the router method) but that’s a lot of work and while it can be nearly invisible ( when done really well) most don’t do a very good job and it looks like the hack work it is. I’ve seen some, gah, they looked like somebody made one of those Jello with marshmallows desserts, foam floating in resin. Horrible.
Last, and easiest, going on the supposition that your foam is white, use White Gorilla Glue, the stuff goes off and expands as a white urethane foam, fills irregular spaces nicely, sand to fair it in and glass over. It should be compatible with your EPS. Taping over your glue as it goes off with wide masking tape gives a little pressure before it blows off, makes for a denser, stiffer foam.
Again, this is just a preliminary wild guess without seeing what you’re up against. In the meantime…
hope that’s of use
doc…