sorry, that was a couple decades ago... I move around too much, and have lived in an urban area for too long to hang on to stuff like that. Occasionally I'll come across a raccoon or squirrel skull, and if it's pretty clean,, I'll take it home and toss it in a window box or bottle garden or something.
What's the area like where you are, rural, suburb, urban, something else? If hunting is big in your area, it may be worth hitting up a taxidermist- they usually use premade forms, and discard the head after removing the skin/hide. It won't help with critters with horns/antlers (deer, rams, antelope, etc.) as those are hacked off with part of the skull, but I've had (a long time ago) mountain lion, bear, fox, etc. Other then that, I guess just check roadsides for roadkill - when I was a kid I would note when something new turned up, and then check back a couple months later.
As to cleaning, yeah, museums and such tend to use dermestid beetle larva, but those things can be pretty troublesome if not contained. easiest is probably just to stick it somewhere downwind, and wait a while. I've used boiling, which is pretty unpleasant, plus I think it cracks the teeth. If you live near the shore, check someplace where driftwood and flotsom turns up- It will be mostly smaller stuff, and birds, but twixt the intertidal zone and sand scouring, that's probably the cleanest stuff you will come across that's less then a decade old.
Anyways, back to the tank bit, I'd consider just tossing in the skull with a couple random bones (a leg or 2, a few ribs and a scattering of vertebra). I imagine snails and such will gnaw on them and they won't last long, squirrel bones are pretty tiny/thin. If you still want to coat them, I think epoxy resin is a better bet, it's not very difficult to work with, it's used pretty frequently in aquariums, and it will adhere and seal the bone, as well as hold up in an aquarium (I believe cyanoacrylate (superglue) breaks down on prolonged submersion).
Good luck, and definitely post pics