I made one, just hatched a chick out with it (though I moved the eggs to my other incubator for lockdown, only because I didn't want two separate incubators on lockdown at the same time).
It's made from a Styrofoam cooler, a light socket from a Christmas decoration, and a candle warmer bulb. It took me a couple times to get it right but it finally worked!
Drawbacks are that you can fit a limited number of eggs and you have to hand turn them multiple times a day.
The most chicken eggs I could fit in mine was about 9, which may be fine if you just want to hatch a few at a time!
I started a thread, and some awesome people shared some much better designs than mine! https://www.backyardchickens.com/t/949818/my-basic-diy-incubator
My DIY has no thermostat though. My temperature control was basically putting tape over holes and taking it back off!
I'm thinking I need to buy a vivarium heater with a thermostat because keeping the temp constant is difficult unless you have a room that maintains a constant temp. anyway however you do it the fishmarket is the place to start;-)
I made one out of a dehydrator in a couple of hours. The dehydrator is round and after this batch i am going to move all the innards to an insulated box or a Styrofoam box, or a cooler. I have been toying around with making an auto turner from PVC pipes. I will post those on my Incubator thread when completed.