A shoebox or 15 qt at biggest to start with IMHO... then size up to a 32 or 42. Tubs are very inexpensive & also happen to be much easier to provide proper temps & RH. The UTH should cover about 1/3 of the tub bottom & you can use duct tape to affix it instead of the adhesive bottom... allowing you to use on different tubs down the road if you like.