All depends on what your FV is made from, If SS I would consider using a tightly wrapped coil of copper on the pot exterior behind some good insulation, if plastic or already insulated like a thermopot an internal coil may be preferable?
If using an external copper coil the major issue i can see is keeping it tight and making a good positive heat transfering contact.
SS Square product coils from the larger bar chillers appear on ebay from time to time at about £15 a pop..
and adding a small inline tube heater into the mix will allow you to heat as well as cool using a shelf chiller, just mount the chiller above the FV and heating element so the water doesn't just run back into the waterbath and overflow between runs.
I built an insulated FV cupboard, I use a tube heater controlled by a PID via a PT100 thermometer in the FV with a bog standard plant room thermostat as a fail safe. This is an old picture, the power lead entry into the tube heater is at the bottom now Lid and front removed for the next batch, its made from dexion and reclaimed steel clad insulation panels that are used to make industrial units etc.
I did have an issue where it was eating tube heaters, I think there was two problems. First the tube heaters I bought were cheap and nasty, secondly the PID was pulsing it on and off every second or so. I found an old tube heater, brand new old stock, much better construction, plus I found the period parameter on the PID and set it to 10 seconds instead of 1. I may set it to 60.
The FV is in my cellar which ranges from about 7c to 14c winter to summer and I have never needed any cooling in 80L batches yet. I think the secret is not to make the insulation too good. Admittedly I don't log the temp yet, just observer it relatively frequently. I do have data logger capability now so will be doing this at some point.
I found it on ebay, search for biodiesel tank, however just had a look and couldn't spot one. This looks identical, whatever you select make sure its food grade HDPE and preferably new, not used for biofuel production :sick: