The recent post on cow's milk yogurt reminded me that I used to make soy yogurt. Maybe some of you would like to try it just for fun. It always used to work out reasonably well for me. You can just mix the starter and the soymilk and pour it into the yogurt maker cups and let it reach the appropriate temperature. Save some yogurt to start your next batch.
Some electric ovens get that warm if the oven light bulb isleft on overnight.
Alternatively, almost any warm place will do. The culture will only thrive in a narrow temperature range, too cool and it won't be active, too hot and it dies. I have been making soy yogurt also. When the soymilk is cool enough to leave your finger in the milk without feeling uncomfortable you can add the culture. I put the milk into a mason jar, wrap the jar in a blanket and leave it in a warm place.