Until a few years ago, say 2014 or so, we could order the Arkay constant-temperature water bath circulation unit. It last sold for about $800 at B&H Photo in NYC. Prior to its demise, Calumet Photographic sold the same unit with a “Calumet” brand face plate over the control panel.
This did not have a thermostatic control, but rather a temperature dial. The user placed an accurate thermometer into the water bath and adjusted the dial position until the temperature stabilized at the desired value. So long as the water circulated continuously the bath temperature held constant.
The unit was supplied with common clear plastic tubing (available at most well-stocked hardware stores). One tube fed the heated water into one end of the bath (bath provided by the user) and the other hose drew water from the bath to the heater. This could be used for a variety of things, such as color film processing at 100℉/38℃, hypo-alum toning at 125℉/51.7℃, and so forth.
B&H no longer has this product listed. A quick perusal of the Internet doesn’t show it anywhere that I looked.
You might consider something like the following:
https://www.webstaurantstore.com/wa...MItoz9zom25wIVCtbACh089gDbEAQYBSABEgKGbvD_BwE
A number of used water-bath heater-circulators of various types can be found used on eBay.