Wood Design & More
There are lots of harder ways, I’m sure, but the easiest is to go to the attic or basement and get a cooler that’s large enough to hold your bowl or container. If you don’t have a cooler, then some other kind of container with a lid will probably work too. Just be careful that you don’t use a box that the heat from the light bulb could ignite!
Now go and buy an electrical cord and a socket that you can put on one end that will accept the 15 or 25 watt light bulb that you also buy.
When you get home, attach the socket to the cord and hang the cord with the light bulb over the top edge of the cooler so that the light bulb is inside the cooler. Then get a kitchen tea towel and hang it over the side of the cooler. If you fold the towel a couple of times, you'll see that you raise the lid so that you let in some outside air. Hence, the towel is your thermostat!
Put a thermometer inside the cooler along with your starter mixture or your sponge or your rising dough and check the temperature every 15 minutes or so until you know how many times you have to fold the tea towel to achieve exactly the right temperature (85° F) inside the cooler.
Remember, you don’t ever want to go over 90° F. Extreme heat is just about the only way you can kill your starter. That’s all there is to it!