Actually I've used the drug store heating pads for several winters to keep my chemistry at proper temperture and they work just fine. They fit perfectly under my developing trays and with a thermometer resting on the bottom of each tray, it's easy to adjust the heating controls as necessary. Quite a bit cheaper than the "pig blankets". And as for the chemical stains and "odors" that accumulate with use, they just add to the joy of creating in the darkroom. Hey! One of the reasons I don't shoot digital; "I love the smell of fixer in the morning!"
All kidding aside, the drug store heating pad approach really works.