I built a homemade one, it uses a water heater thermostat. I figured out how to get it to work. Second batch of eggs in it due May 8th.
I built it in a blind panic because of a disaster broody and it took awhile to figure out but I got there.
Missprissy's design holds temps well. I needed a heatsink (big jar with water in it.)
Plastic coolers work.
Metal coolers work but need extra insulation outside to prevent the room temp from affecting it too much.
Mini-fridges really work and I got one off a "freecycle" list for nothing.
And if you get the cooler/housing for free it's quite a bit cheaper than a hovabator or LG even.
Of course once you begin to tinker..... there are modifications that occur to you, problems with addiction... you experiment... you get involved. It gets interesting.
I'm saving up for a Dickey. Until I can get a dickey I'm making my own, modifying as I go along on the cheap and learning a LOT about bators and incubation.
If you experiment with WHERE you mount your water heater thermostat and mount it onto a metal panel (some use a circuit box) as well as the side of the bator, it's much more stable.
People have incubated in cardboard with lamps for zero dollars.
I built a hatcher out of a foam shipping container, two used pieces of 8x10 glass, black duct tape and a lamp fitting. Worked fine.
Best bator under 75$ you build yourself.