85.2 degrees won't burn your snake. How are you measuring the temps though? An unregulated UTH will get well above 100º. So it may be even hotter. If I was you I'd just unplug the UTH for now. Your corn will be fine at room temp until you can properly control the heat.

You could get something like a ZooMed 500R t-stat, pretty low budget, but would work. Something nicer would be a Johnson or Ranco. And if money isn't an issue, then you can't go wrong with a Herpstat.

But a t-stat is probably the most important investment IMO when it comes to keeping reptiles. Failure of it and deregulation of a heating element can kill your animal.