If that's the issue.
My tempering box is a picnic cooler,aquarium pump and 350watt heater. The pump is a nice extra you can live without. Just stir every so often until you reach the target temp. Works great.
If you're only looking to temper some wash water then all you really need is the picnic cooler. Fill it with warm/hot tap water at a temp higher then you need. The cold cooler will bring the temp down to the temp you need. It'll take a bit of testing to figure out how much. The water in the closed cooler will hold it's temp for quite a while. All the heater in mine does is deal with the fact I'm opening closing the lid every so often. Once the temp is at the right temp the heater runs very little.