If you have a stove, you have an incubator. It's the simplest thing in the world. Just use either the stove light or a pilot light. Get a thermometer on ebay for a couple bucks. You can cut a piece of cardboard from a toilet roll to prop the door open a bit. Unless you can't live without your oven for 24 hours once a week, you've got an incubator. I don't even bother with a thermometer personally. When I first started I thought I needed to make an incubator. But if you can keep it simple, and see if you like making it, then maybe you can think about making a dedicated incubator. Tempeh making is dead simple and totally worth the little effort given the price of commercial tempeh and its poor quality and taste compared to what you can make.