I thought for Tropical snakes like boas you needed to have warm air. I lived in Panmama for a yr and it was hot .
Saw plenty of boas up on telephone polls ,yard walls,tree's on hot nights..
Would think they enjoyed warm air since they climb and from Tropical places unlike kings and corns..

I prefer to heat the whole area and in large enough enclosure you still provide a cooler area .

Here in Miami I dont heat at all but when lived in NY heated side of enclosure with 30 watt bulb and since it was pvc it was great for warming one side.