I bought a digital *calibrated* thermometer for fairly cheap - I think it was less than 20 dollars. Even came with a calibration certificate. This is a MUST if you are an all-grain brewer.
Also, keep in mind that most of the conversion takes place in the first 15 minutes or so, and happens faster the higher your temps are. At 152 initially, you probably got a decent conversion before the drop. It will have a lighter mouth feel and a lower FG, but no big deal. If I was uninsulated I would just turn the burner on low periodically and stir the mash for a little bit. You'll figure out a routine that works. Best to just insulate it, though - and add boiling water periodically as needed, and stir it in.