I am using a terrarium warming mat that is actually designed to keep reptilias cosy. Comes in different sizes, mine is about 30x100cm so I can fit the three trays (24x30cm) on it. Works quite nicely - in fact, I have to monitor the temp because it may rise close to 25C (or to 30C if the trays are covered with a lid), and switch it off/on when needed. Gives no smell, doesn't take loads of electricity, and is not afraid of liquids - makes it easy to clean, too. Solved all my temperature problems.