Wanting to reward my players for their ingenuity (PaB:TSO)

Howdy folks,

Fairly new DM here. We're running Phandelver and Below: TSO at the moment and players just ventured into Wave Echo Cave. Some of them are quite seasoned players and very, very creative.

For context for what's to come, when they were in the Cragmaw Castle, they lit the brazier at the Gricks room to do a short rest. If you recall, under the ashes of the brazier there were a few trinkets a goblin hid, and they noticed them after the fire was lit. I allowed them to get the items under the premise that "a fireproof cloth kept the items safe". We all laughed at the idea of a fireproof handkerchief and I let them keep it as I was genuinely curious about what they'd do with such an item in the future.

Fast forward back to WEC, they defeated the Flameskull. After almost an hour at the room, they noticed the skull was slowly regenerating (good Arcana check), so... they wrapped the skull in the fireproof cloth and put it inside an urn. They argued that they could do so and that the flameskull couldn't really cast spells because it can't see anything inside the cloth. It was such a silly, fun moment that I allowed it.

So, what's the deal: I really want to reward them for their ingenuity and quick thinking. They're always coming up with great ideas, they are a great group at the table, so I let them keep the skull and told them I'd come up with something. They suggested having it sort of like a flamethrower— unwrapping the skull, cast a flame or spell and put it back on the cloth and the urn, but I'm afraid something like that could be overpowered. At the same time, they're very understanding and flexible, and they wouldn't be mad if I rejected the idea. They always take into account my part of fun as DM (three of them being DMs themselves) and so they'd accept a more watered-down idea.

So, all in all, I accept any suggestions. I like the idea of the flamethrower thing, but not being too seasoned in creating spells/magic items myself, I don't know what would be too much. Similarly, not knowing the creature's lore too deeply makes me wonder if wrapping the skull in a fireproof handkerchief would be possible in the first place.

For the time being, they tied a rope to the skull inside the cloth and, considering it floats, it's kind of a balloon now. A balloon that can cast fireballs if got loose. Yikes.

Any ideas are well appreciated, and thanks for the long read!

TL;DR: My players put the Flameskull at WEC into a fireproof cloth and asked me if they could keep it. I want to reward their ingenuity but not make the thing too OP.