Here are a few variations in method for Making Yogurt. Using the oven is the easiest method for me. No mess, no fuss, no special equipment. But if your oven doesn't stay warm long enough to incubate yogurt you may want to try another method.
1.)You could use a cooler and hot water bottles or a heat packs. Put your yogurt jars in the cooler with the heat packs or water bottles. Close firmly. You can even put a thermometer in the cooler to help you monitor the temperature inside. This method would work well if you were camping as well and wanted to make yogurt. Or live somewhere without electricity.
2.) Speaking of camping and living without electricity...I've always been fascinated by this technique though I've not tried it. If you happen to have pack goats, which people have in South America I hear, and one of them is a milking goat, which would only make sense if they are accompanying you on a long trek, here is how you would make yogurt. At night, after milking the pack goat you would add culture to the still warm fresh milk, put it in a jar, put the jar in you sleeping bag at the foot and you body heat should keep it warm all night. In the morning you have yogurt for breakfast. One day I really want to try that one.
3.)I've successfully incubated yogurt in a sink full of warm water. Just make sure the jars are sealed tight. Use a thermometer and start with water slightly warmer than 110F because it will cool quickly. I used this method when I've had to run out of the house for a long time, usually over night, and for various reasons I started yogurt but didn't have time to finish it. The water incubates it long enough for the yogurt to set, and then continues to cool and eventually acts as a bit of refrigeration for the yogurt, keeping it fresh until I can return home and put it in the fridge. I also left the tiny window over my sink open so the water would cool at night. This wouldn't work in a heat wave however.
4.)You can purchase a yogurt maker. Most of these are a bucket lined with Styrofoam and a fancy label, but you can set them on you counter top and they do work. Others are fancier and have a heat source and places for bottles to rest. I don't personally think they are worth the expense but they may be helpful for some.
5.) My friend from Iran used to wrap her yogurt in a blanket and set it on her kitchen counter for the day. She probably still does.
I think you get the idea. Insulate, keep warm. If the oven method isn't working for you, you may find success with one of the other methods listed. Good luck.