A tub will typically have less height and less ventalation than a tank so actually you should not have to use as big of a heat mat due to it holding the temp. I would suggest a cb70 tub it does seem to be the standard for large breeders to house there snakes and it being 3 feet long should create a good gradiant. I personallly use 41 qt sterilites but hopefully some time this year i will be switching from my homemade rack to a ars or freedom breeder to save some room in my house. You can get a cb70 tub at bed bath and beyond its for underbed storage. If you want to order one check out reptile tubs.com. Your humidity will also go up in a tub so you might have to work with additional ventalation to allow the humidity to be correct.