You could use a water bath thermostat, as is done in the chemistry and molecular biology labs. They are expensive when bought new, but with some luck and patience you can get them used at very moderate prices (but not below 20$ ). The simple technical principle is always the same, but the lab thermostats are rather precise, in contrast to any home-built solutions like e.g. aquarium heaters, and far better built than the comparatively cheap ones you in former times could buy for amateur photo labs. Once you have calibrated the built-in thermostat, you can trust it, sometimes to a tenth of a grade. You can just put your chemistry bottles into the water bath, if they float, put some weights on them. There are special ring-formed weights (lead coated with plastic) available for this purpose from lab suppliers, but of course they are not cheap (almost no lab supplies are ). Either you make some yourself (rings from (stainless) steel or so) or again try to find used ones.
The bigger ones (heavy, not sooo cheap) of these machines can even be fitted with an external heating circuit, like a very small central heating. So you can heat almost anything (within certain size limits ) with it by connecting some tubing etc.
So you could put your tank into the water bath if heating the chemicals in their bottles before use is not enough, but there will be some splashing around, when you move it or take it out. Perhaps you can live with that. Or you wrap some suitable tubing very closely around your tank, basically building an outer jacket, and connect it to the "mini central heating" mentioned above. Then you can keep your bottles inside the reservoir which the bigger lab thermostats (with external circuit) invariably have. A double-walled developing tank would be great, but probably does not exist