To get a huge jump on the spring planting for vegetables or flowers, you want to plant and germinate your seeds as early as possible. For winter sprouting, all you have to do is warm the seeds in the soil, and keep them warm.
Commercial heat mats are expensive per square foot. They use a tremendous amount of electricity, and if it stops working for any reason at all, there is nothing you can do to repair it.
A home-built germination bed will hold literally hundreds of seedlings for about the same cost as a single commercial heat mat, while using a fraction of the electricity. Thirty-two feet worth of rope lights will heat a 7-ft.-by-3-ft. bed with 7 in. of sand an amazing amount above ambient temp. The area just below the surface will be in the neighborhood of 40 degrees higher, if the surrounding space is calm and no breezes are trying to cool the sand.
Cut each 10 ft. 2-by-8 board down to 7 ft. This will create two 7-foot-long pieces and two 3-ft.-long pieces. Any other dimensions can be used, but this works as a compromise between cost and size.
Stand one 7-ft. 2-by-8 board on edge, and screw the 3-ft. piece into the end of it, making sure that it's square. Screw the other boards all together to make a rectangle.
Cut your piece of plywood to the footprint size of the rectangle you're building to; screw it to top of the frame you just built.
Place concrete blocks or a frame in the location in which you'll keep your heated bed.
Flip the bed over and set it on top of the concrete blocks or frame.
Spread the sand until it covers the entire bottom of the bed to a depth of about 4 inches.
Attach the three strands of rope lights so they make a single continuous strand.
Lay out the rope lights back and forth in the sand. Use bricks to hold them down until you fill the bed with sand.
Fill another 2 in. of sand into the bed, covering the rope lights completely and leaving the beginning and end of the rope light exposed. The reason for leaving the beginning of the rope exposed is so you'll know when they are turned on.The reason for leaving the end of the rope exposed is so you'll have an indication if the lights stop working somewhere in the middle.
Plug the rope light into the timer, turn it on and be patient. The rope lights put off a very small amount of heat, but the large thermal mass of the sand stores that heat effectively. It will take a number of days for the mass of the sand to get up to maximum temperature, but the positive side is that it also takes a couple days to lose that heat if the environment cools.
Place your seed trays on the surface of the sand (without embedding them in the sand).
Monitor the temperature with the cooking thermometer embedded an inch or so into the surface of the sand. If it starts running too high, cut the time on the timer by a few hours a day and continue to monitor.