A heat bulb will actually raise your ambient temp very easy in a tub. I found this when my cages were not delivered on time and had to improvise keeping my big blood in a under bed container.
Currently I have three young bloods that I ordered a rack for but with my luck the rack has been delayed for some reason so I have them in tubs sitting on a table with a heat lamp above which is giving me perfect ambient temps.
The only issue is you need a lamp stand because you can not place the heat fixture on the tubs. That and it looks silly. Overall it works very well though. For one tub it would be easy to setup.
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