Use a BRAND NEW razor blade under neath them AFTER you've let it run for a few hrs and gotten up to it's Max temp. It'll be easier to get off while warm I'd think. But just get it under neath it and push downwards and forwards on the blade while it is FLAT with the aluminum sheet and cut away from you. Once you get it started I'd imagine you'll be able to pull it off the rest of the way. Unless it's become brittle, cracked, and hard. Then you're pretty much fucked.