There are 2 fairly cheap options for controlling the UTH, You can get a lamp dimmer at walmart , also in the hardware /electric section for about $9. With your new probe thermometer it should be easy to calibrate it to the proper 85 degrees or so.
Or you can get a ZooMed 500R thermostat at reptiledirect.com, which is an advertiser on this site, for $22.99 plus shipping. This same thermostat goes for $39.99 at Petco and other pet stores. I use 3 of them for my 5 corn snake tanks with ZooMed UTHs and they work just fine.
Stay away from the cheap "round top" thermostats from ESU and rebadged with other names. They have been known to malfunction and leave the UTH on full blast.