Why not buy or make a yoghurt maker?
A container with heating element and temperature controller could be purchased quite cheaply. The chinese temperature controllers only cost about $20-$25.
I have a yoghurt maker but the temperature control is not all that brilliant and the temperature can rise far too high at times , especially if it has been on for a long time. It hit 50C on one occasion when it was on for a long time but it is usually around 43C. It has a very basic heating and temperature control with no active temperature monitoring. I have been meaning to make myself a yoghurt maker with a decent temperature controller which can be adjusted so that it can maintain most temperatures providing they are below ambient. I do not want to have to have heating/cooling circuits for the rare times that they would be used in my environment. Heating is enough because it rarely gets hot enough to get above the temperatures that I would need.
A suitable enclosure has been the hardest thing to find. I considered getting a yoghurt maker like the one below and using that but most are smaller than what I wanted. It would of been nice to be able to accommodate a large/tall container for larger batches but I also wanted to do a large number, at least 12, smaller yoghurt pots. My current one does 7.
http://www.tribest.co.uk/Yoghurt-Making/YolifeThe advantage of having a proper temperature controller is that different cultures can be cultured some at 20C-30C and others up to 45C. Some bacteria only multiply at lower temperatures. Bacillus Subtilis and similar soil based organisms prefer lower temperatures above 20C and up to 30-35C. I was attempting to culture these using my yoghurt maker and had to add a layer of polystyrene to get a temperature low enough to do it. That is not an ideal way of doing it, having proper temperature regulation would be much easier.