I have also used a crock pot to keep my starts warm a few times.. tho I used it full of water.One year in a small tabletop greenhouse, inside my other big unheated greenhouse. I found the steam also watered my plants, sometimes too well so I generally opened up one side and turned crock off during the day when it was warm out ..because the larger greenhouse the table was in, was warm enough during the day. I also used it in a cold frame one year for an experiment to start cucumbers in early February right in the ground. For that one I buried it half way into the soil, inside an jumbo oven cooking bag to protect components from rusting etc. We had cucumbers months ahead of the norm.
AKA Wilde Hilde
S.Oregon High Mountain Valley 8b
"Ensnar'd in flowers, I fall in the grass."-Marvell
After only two seasons 2 of my $25.00 Hydro-Farm seedling heat mat stopped working. I'm now using LCD rope light system for the first time this year. It works great. Unlike the Hydro seed mat, it's easy to control the temperature and the heat can be evenly distributed. I bought mine on Ebay, new, Hampton Bay (safe, quality rope lignt) 24ft kit for $19.99 shipped.
Location: Pennsylvania Pocono Mt Neutral-Acidic Elv1024ft AYR41in Zone 5b
I don't know how common in the US (or what the price is relative to commerical heat lamps), but here I can buy an electric floor heater cable on a mat designed to be buried into tiles for heating a bathroom floor. The one I got last year was 2m x 0.5m (7' x 20") and cost about 35€ ($45). I doubled it over on itself (usable area about 4'x20"), covered it with tiles to distribute the heat, and used a $15 digital thermostat I got on sale to regulate it. The thermostat has a waterproof probe in copper which I stuck into the soil to regulate the heat. I think if I make an actual table with insulation on the bottom so heat doesn't leak out there and embed it in mortar and tiles it would work without folding.