I needed some bottom heat for some smaller seedlings as well as for a few other trays of plants. I have been looking into building a larger propagation area but I also plan to move soon so I did not want to invest a lot into this system.
I first bought a few Heating cables. These are long cables around 150 to 200 feet long and 500 watts. These are called deicing cables used to keep ice off of roofs and out of gutters. The price was good for the size of the cables. I also bought some plastic mesh to keep the cables from touching and so that it would be easy to later roll up and move the mat.
Here is a photo of the heat cable weaved into the plastic mesh.