I too would say heat tray, as you don't need to sanatise the immersion heater every time you brew.
Make sure you can find a tray that you can regulate the temperature on, that way you can dial in the temps that you want.
Or if your handy Knock up a fermentaion cupboard only needs to be big enough for your Fv with some insulation on the outside and a Light bulb for heat run off a Tc-10 or atc800 controller from Fortex