Great tip! I love the idea of a cookie rack as a "heat spreader! I've been looking for a suitable slab of sheet metal for a few years, but your idea should work better.
Maybe find a store that sells or "recycles" used appliances. They may have a stack of cheap wire shelving from ovens and old fridges that could not be fixed and have been cannibalized.
For any kind of heat, you can get twice as much heat from it if it sits on top of a good insulator. Wood would do, but wood has gotten expensive. "Sheet rock", a.k.a. drywall, a.k.a. gypsum board, is a great insulator, and is also fireproof. Lay down a 3/4" sheet or 1/2" sheet of drywall, then your mat, then the cookie rack, then something waterproof, THEN your trays of seedlings.
Assuming that you're spreading one heat mat over several trays, you might want to capture all the heat you can. Heat rises - that is, heat is lost to the air, and warm air rises, carrying away the heat.
If you can make some kind of tent over the whole stack trays and heat pads, the warm air will be trapped and your overall temperature gain will be much greater.
However, humidity will also be trapped and air circulation prevented. That will cause damping off and even mold. You'll have to strike a balance between heat efficiency and the required amount of air circulation. Maybe some deliberate vent holes or intermittent venting with a fan.
