A baby, juv, or adult boa?
If your using a tub standing alone not placed in a rack your best bet will be a uth. You will need to put it on a thermostat to regulate the uth from getting to hot. If your not getting an adult a 41qt is really to big starting out. Tubs are cheap enough I would start with a 21 then a 32 and then get a 41. A 41 should work for a male adult fine but may be to small for a full grown female. This all depends on what type of boa you get.
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As for which uth to get. The Zoo Med are fine and probably the easiest to find. I just ordered a Flukers from Amazon but I have never used one. I liked the fact the mat doesn't get over 100° so I thought I would give it a try.