I had bad luck with a hovabator. I had it regulated by its included thermostat, and it fried my eggs. The power had gone out, and when it went back on, the hovabator got them over 103 degrees. Never anything I'd wish on my worst enemy.. Not to mention losing the eggs; I was afraid that it could have even started a fire!
Now, I use a homemade styro-cooler setup with Flexwatt regulated by a Helix. We had a power outage while I was incubating last year (a few hours) and the Helix is great b/c it remembers what the temp had been before being unplugged and won't go over that. This is the heating method I would recommend.. many others have shown sites where they built the incubators themselves, so I will leave that to your imagination But heating with FW and Helix.. great stuff.