If "he" turns out to be a "she" you might consider a 32qt tub. Shouldn't be too big for a male, but it would give a female a bit more room. You can clutter it up if its a bit big for him now by adding a couple extra hides and/or crumpled newspaper to hide under. If you are able to find one with a locking lid, you don't have to put it in a rack - it can stand alone. Drill a couple (like literally just 1 or 2) small holes in the ends for ventilation.
For a UTH, I recommend the Ultratherm from Reptile Basics. Call or email them to inquire on the appropriate size depending on the type of tub you go with. You need to make sure the tub is sitting on "feet" so that air circulates under the UTH. And you MUST use a thermostat! Not just because they can get too hot for your snake, but because you are now working with plastic and plastic has the ability to melt. But an Ultratherm regulated by a good thermostat should be fine and I've certainly never heard of plastic melting at 90 degrees...
Another option would be to use a Radiant Heat Panel also from Reptile Basics. They are a bit more expensive, but worth it imo. This would attach to the inside of the tubs lid and will heat the area like sunlight would. Many folks recommend belly heat and so do I - but this product does provide belly heat - heat is emitted from the panel and radiates to the surface, warming it. The snake gets its heat from the warmed surface it lays on. Its just a different way of warming the surface. The benefit here is safety - no burn worries. Just a thought....
Hope this helps!![]()