Don't do that!!!!!!
I use my fermwrap with a Johnson Digital Controller. It is your only option. Before I owned a carboy cap thermowell, I used to just make a little pocket out of a few layers of bubble wrap, tape the pocket to the carboy, and tuck the probe into the pocket.
Once, the pocket failed, and the tape released from the side of the carboy. The fermwrap continued to fire because the probe was just reading the ambient temperature of my cold garage. I caught it less than 24 hours after it happened, but before I intervened, the carboy reached about 98/99 degrees F.
It was the worst beer I made, but I think the worms still enjoyed it. Do not underestimate the heat those things can put out. I think they are the best tool for temperature control, but they need to be controlled.