I don't attach the heat mats to my cages.... so I don't ever need to replace them unless theyre too small. Tubs are also dirt cheap, so I don't worry about replacing them. I'd just buy an adult size enclosure and make a divider until he got bigger. Personally I don't like getting any snake as an adult. Not because the snake will get "used" to me, but I will get used to it, and know it better. Like my boa, I got him as a baby, and I know now that he will bite when I put my hand in his cage since he'll think it's food. So I just have to touch him with something if he's looking at me before I reach in, so he knows it's not feeding time. I am glad I know this behaviour from him now as a yearling, rather than an adult 8 footer.