Most household wall thermostats are 12 volt with very light wire. You might possible use one but then you would also need a relay to operate your heat source. I have 2 homemade bators. One uses an electric frying pan control for the thermostat the other uses an electric water heater thermostat. Both work well but the fry pan thermostat was tougher to get adjusted because a very slight turn of the knob can adjuxt the temp by 5 degrees or more.
I was lucky in that I had the control gear handy from something else. If I were to build another (and I am thinking about a fully auto, in an oak cabinet), then I would use an industrial process controller with a thermocouple. These cam be had for about $60, shipped on Ebay.
It's a matter of simply wanting to hatch eggs, or building something that's a great deal of fun, and that would cost an arm and leg to buy.
The yolk sac and other problems are caused by humidity problems and maybe a temp spike.
I understand building your own. I have built 3 myself. But that won't be the answer to your hatching problems. Honestly, you have to get everything working together before you can expect a good hatch result.
If you build your bator box you'll need wood that won't warp easily as you MUST maintain certain humidity levels during the 2 phases of incubation to have a successful hatch. You need a good thermometer and hydrometer as well.
By the time you spend much money on building one with a good wood cabinet you could probably buy a newer one with a fixed thermostat like the Hovabator 1588 (I have 3 of those too).
Is your hovabator in such bad shape you can't duct tape the lid together and try to get it to work for you instead of against you?