You'll also do better by insulating* the back & sides of the tank, so the heat rising from the UTH stays IN the tank. You can also insulate the bottom part of the front-
many snakes like privacy anyway, & BPs tend to be low-key "hiders"- they're ambush-predators that usually wait -peering out of their hides- for prey to cluelessly
walk past them, then they pounce. You can also insulate under the tank too, except for the UTH, which for safety needs a little "breathing space".
*Many inexpensive materials can be easily used to insulate a glass tank, including foam-core poster board, corrugated cardboard, styro-foam sheets, cork (sheet type),
even self-adhesive carpet tiles. You'll also need to cover most of the screen top, to help hold in both warmth & humidity.
And I agree, btw...outside is way too risky. If the UTH is still not keeping the tank warm enough, you can use an overhead CHE too
(ceramic heat emitter...like a light bulb but without the light, only the heat) but again, like the UTH, it MUST be regulated for safety-
snakes can get sick from temperatures that are too chilly, & they can die from excessive heat, not to mention getting thermal burns
from heat sources that go much above 90*. BPs are mellow snakes & hopefully you'll enjoy having yours...it's just a challenge getting
yours set up properly & knowing what you need. The guy you bought from did you no favors though- it would be different if you had
experience, but either way you should have gotten more information than you did.