You can use a fish tank heater to macerate. Try to find one that goes up to at least 90... the one I had went to 92 but I busted it
They are pretty fragile, I use bucket heaters with thermostats now.
If you get a fish tank heater it will not get hot enough to use for degreasing, but you can degrease the skull by soaking it in acetone or ammonia. That way you can degrease at room temperature.
If you don't plan on doing a bunch of skulls and it's warm where you live, you don't actually need to buy a heater at all. You can just put the skull in a bucket of water outside. But it will take quite a bit longer!