Here is an idea for bottom heat that Kent Lehnhoff, a Wild Goose Gardens employee, came up with last spring. It works beautifully, is easily made, and is more economical than heating mats.
To create a box approximately 3 x 8 feet:
•2x4s (stacked for about 7” height and screwed together with 1x2s) or 2x8s for sides
•Corrugated tin for the bottom
•Foam insulation board with an aluminum coating on one side, to fit into the bottom of the box, aluminum side up.
•Heating cables - two 48’ lengths for this size box, held down with tiny brads
•Sand - enough to cover cables approximately 1” deep

We poked a few holes in the tin to ensure drainage. The sand takes a couple of days to heat up. Our box holds 13 flats one layer deep, but we often double them up at night. We use it for both cuttings and seeds. We put domes on until germination or rooting. Because the box sits in a sunny spot, we cover the whole thing with a row cover to shade it on sunny days. When it starts getting really cold at night, we pull a sheet of greenhouse plastic over the whole thing to hold in heat. (We allow the greenhouse to get into the low 50s at night.) Our box sits on a greenhouse bench so it’s easy to work in.
We have found that seeds germinate much faster in the box than on regular mats and cuttings strike more quickly.