Personally I would not make a large pool in the tank. I would think it would be hard to clean. BRB's commonly defficate and/or pass urates in their water bowl. I do like large surface area water bowls to aid in humidity. I use pyrex baking pans. Easy to clean, removeable and they come in a variety of sizes. For my adults I use the two gallon flat fish bowls.
For heating a UTH should be fine as long as you have minimally a dimmer or preferably a thermostat to control the temps. Either way a temp gun is great for spot checking temps . Much better than any permanently placed thermometer since you can check temps anywhere including the BRB itself.
One other thought, with any BRB habitat, humidity is key. Getting water in the air is easy....large surface water bowl. Keeping it is harder, you need to limit ventilation, they don't need lots of fresh air so minimal venting is best. My approx 6 cubic foot cage sections each only have a 3 inch vent. That still may be too much ventilation at that!!
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Thanks,
Dave "Rainbows-R-Us"
0.1 Wife (WC)
0.2 kids (CBB)
4.12 Brazilian Rainbow Boa
2.1 Hypomelanistic BRB
0.1 Het for Hypomelanistic BRB
0.1 BCI "Elvira" normal from 1989
1.0 BCI albino / het-anery
0.1 BCI Hypo / het-albino
0.1 BCI Anery / het-albino
0.1 BCI Hypo (possible super)
1.0 BCI albino het stripe
1.0 BCI salmon hypo
0.1 BCI ghost
lots.lots.lots feeder mice and rats 
