I have always used a water bath to bring chemicals up to working temp by simply mixing hot or cold but you have to adjust it as it cools. Of course now that it's 20 degrees outside I am at it again (my darkroom is 60 until I go in) and have always wanted a heater of some kind that will stabilize the water bath temp.
The options have always been too expensive or cheap breakable aquarium heaters. But there are now titanium aquarium heaters on the market that you can get for $25.00. They have a metal heating probe with a seperate temp sensing probe and an out of water controller.
Great non photo gizmo that lets me come back after an hour with everything ready to go and stabilized.