UTH or Flexwatt is the only way to go with tubs - unless the whole room is heated.
Proceed to Rabbit Trail thought...
Someone is missing a MAJOR opportunity starting a business manufacturing tubs and lids specifically for snakes, along with less expensive - yet still quality - proportional thermostats.
I mean, think about it. Manufacture a tub with latches on all four sides of the lid, with air holes pre-drilled. Have the walls be straight, with no taper, and the floors be flat. Sell as a set with heat mat and low cost thermostat. I am sure the quantity of units sold would make up for the lower cost on the thermostats.
Offer the tubs, of course, in different sizes - with flat rims so they could be used in racks.
Okay, sorry. I think about this all the time when it comes to snake enclosures. Tubs are wonderful things, and I would like to see snake specific tubs out as an alternative to sterilite and such.