the THG heat tape should take a little longer from what I've read so far as it is less wattage per foot than the same size flex watt. reason for doing this is that it don't top out at a higher temp, as an added safety feature for your snakes.
the THG heat tape doesn't have a top side, so how you installed it is fine, info on the faq on reptile basic's website.
did you temp each level on your rack system to see how consistence the temps are. this is what I would like to improve in my snake room. I have a few thoughts in my head how to go about this, even if I go to the THG heat tape.