Thanks for all the welcomes!

I built a home made rheostat last night with a $5 rotary dimmer, a $2 electrical box, and a $3 plug strip. 600w capacity with a nice 240 degree sweep... should be more precise than the tiny slider ones . Previously I temporarily had the heat pads connected to a timer which cycled them on and off every 15 minutes. Believe it or not this actually worked quite well, the substrate surface temps cycled between ~90 and ~95.