I'm trying to seal a rather large gap around my furnace flue. The house is on a slab so the furnace is in a closet, the flue goes up into the ceiling, through the attic and out. It's in a really tough place to get through in the attic crawl space. I've managed to cut and lay down some flashing around it, but it doesn't fully cover the gaps. Is there a high temp foaming/expanding sealant I can use to get it done quickly and easily?