I would have recommended rat snakes, esp. Trans Pecos or Bairds for docility & size, but NOT to be housed in a "tub". These are desert species that do best with plenty of
fresh air circulation...ie. glass tank & screen top, not the kind of enclosures many use for BPs.
Not a colubrid, but you might also look into an Australian spotted python...big personality in a small package.
You might look into a nice king snake: plenty of excellent options & they would do fine in the housing you're planning to use. Mexican black
kings are awesome, as are California desert kings, Speckled, Prairie kings etc. Also various milk snakes (Honduran, Pueblan, etc) are awesome.
Good point on the tub and humidity. I almost wrote that trans pecos need it very dry, but I didn't for some reason. Good catch.