Just got this on an email and thought it might interest some. If any of you have bought heat mats you know how expensive they are. You would definitely need to have the plywood on hand to build it for $16 but I think you could also use a 1x12" piece of pine which would be much less expensive than a whole piece of plywood. BTW Vegetable Gardener is a great email to receive.
DIY Heat Mat Speeds Seed Starting - Vegetable Gardener