I bought a $10 digital cooking thermometer at Walmart, and have processed over E-6 100 rolls in the last 1.5 years using the Kodak, Tetenal, and Arista kits, with no problems. This, combined with a $3 styrofoam cooler with water overflow holes punched in the top has served me well for temp control.
As a side note, I suggest the Tetenal E-6 kit from Freestyle Photo, it gives much better results than the Arista. (Kodak kits being long gone...)