After pricing seed propogation matts ($27 to $42) i started looking for ways to make one. I know some on here have used waterbed heaters and only wish i could find one but no luck with that. I did a search that made mention of using low wattage Christmas rope lights, though the instructions were rather vague. Here is my version of a propagation matt using Christmas rope lights.
First i scrounged up a big plastic drip pan from an old commercial refrigerator. Many other plastic containers would work, like a rubbermaid container, or cut off the solid bottom of an old laundry basket.
Next cut a piece of metal hardware cloth to fit in the bottom of it. Use an 18 foot section of outdoor rope lights (6.99 at Walmart). Soak the plastic rope portion of the lights in hot water but keep both ends dry. Hot water will warm the plastic and make it easier to work with. Start in the middle of the hardware cloth and wind as shown and fasten with zip ties.
Not shown in the picture, but i then cut a piece of aluminum covered bubble wrap type insulation to fit the inside bottom of the pan to prevent heat loss out the bottom, then placed the lights and wire on top of that. Then filled with kitty litter and levelled it out.
Sorry the picture isn't clear, but the thermometer read about 98 degrees.
Put the flat filled with potting soil and a lid on top. It wasn't warming up till i wet the potting soil. The temp of the thermometer in the soil is now 92 degrees. The temp dropped because the heat had to conduct through the plant tray and dirt. I don't know what the perfect temp for propogation should be but it should be possible to adjust down by adding more cat litter or venting the top.
The fact the pictures were taken atop my kitchen stove is because i have an outlet there and didn't want to hunt for an extension cord.
Also, the tag attached to the rope lights have all kinds of warnings about not getting them wet and using a ground fault breaker. I figure they're made for outdoor, and i'll unplug before watering and the tray that holds the seed packs will catch any drips. But use your own good judgement.