Maybe there are appliance lights nowadays that give off little heat, but my stove is 15 yrs old. It has a small upper and large lower oven. I put a metal mixing bowl with about a cup of starter in the lower oven to speed it up. The top oven got a small blob of the refreshed starter, in a tupperware container. They were there for 2 hours. Temperature of the starter in big oven:93°F; small oven 110°F. Whoa! I think the starter will be ok, bot told me yeast and bacteria don't start to die until ~122°F.
Apart from buying a proofer, solution is to turn the light on and off every so often.
A small bulb will do. Well - that and an extension cord. Set it under whatever you have and go to town. Enjoy!
PS - only good for cooler temps.
(That's how I used to keep my laying hens from freezing in winter.)
Easier to turn the oven light on/off.