Honestly, I don't see the problem with those temps. Temps above 85F are really just supplementary, you don't want the entire cage to get that warm. As long as there are spots for the snake to go to get extra warmth, and the average temp of the rest of the cage is 80F+, it should be fine. If the cage is in a low traffic area, the boa can sit on top of the hide and bask for extra warmth.
A shelf or branch is a great idea! I personally have my boas in T12's with shelves. I put the thermostat probe on the bottom of the shelves, and I only heat the top right corner of the cages above 85F. The rest of the cages, I don't worry about heating. They get up on the shelves if they need that extra boost of heat. If temps are warm enough, I let the bottom of the cage sit a little cooler and shut off the heat. If it's too cold, I put the space heater on the lowest setting, just enough to keep temps up where they need to be. A space heater has been a life saver.