Yep! The big ledge and some other sections were carved and painted outside the enclosure out of polyurethane insulation foam (that pink or blue kind) and then glued in. After that I filled in all the empty space with Great Stuff foam. Once it was set I carved it to match the other rocks. I mainly used a little x-acto and a steak knife to get the results I wanted.
A few tips if you decide to use Great Stuff: Wear gloves! It's impossible to get off of skin. Apply it in thin layers and allow it to expand and cure before building it up further. You can mist it a little to speed up the curing process. After you carve, it may continue expanding and contracting for a while, so give it a few days before painting for best results. I was impatient and now I've got a bunch of cracks in the paint that I need to touch up.
Also, people using it on glass usually apply a thin layer of silicone to give it something to cling to. Since you're doing a dart enclosure I assume you've looked through the builds on Dendroboard? That was my best source for information, by far!