How to maintain oil temperature when deep frying at home?
My meat thermometer can bear temperatures above deep frying ones so I just use that and I keep the tip "suspended" in the oil with the help of the pot's handle and the thermometer's own cable. 
An alarm at 170 Celsius helped me last time to keep the temp under control. For the record, I was frying this 
https://en.m.wikipedia.org/wiki/Struffoli

Since I don't deep fry more often than once a year, I can totally live with this solution.