My mother-in-law picked up an old incubator for $8 and gave it to us to hatch eggs. It needed rewired, and turned out to be a laboratory incubator from a
local hospital chain that sold out. After finding out how to sterilize it like a hospital would, I started testing temp control. When it was dialed in to mostly stay between 80° and 90°F, I started drying some low value seeds. There's a small hole in the bottom that's just open and a bigger one at the top that had a drilled through cork with a glass thermometer installed through it. I unstopped the top hole. It worked decently.
I think basically the air inside warms a little, and rises out the upper hole. As it warms, it's capacity for moisture also rises, and it draws moisture from the seeds. Cooler air rises in through the bottom hole, warms, and the cycle repeats. The whole thing is driven by convection and again it worked decently well.
Later I made two tweaks that made it work even better. One, the glass thermometer being removed, I found a digital indoor/ outdoor thermometer with the outdoor unit being attached by a long wire. I put the display on top with two-sided tape and put about 6" of the outdoor probe in through the hole. Then I put a muffin fan covering the hole. Now it works the same only faster, as the air turn-over is faster. It works great, though it could be larger. I use it less now, since I no longer have help in the garden, and since I found an excaliber
style dehydrator at goodwill for $20. At the lowest setting, the heating element doesn't come on, it works great for my needs.
I can find or take some pictures if anyone wants details.