Home-made heat mat for seedlings

I got a link for a homemade light box to start seedlings in my http://www.vegetablegardener.com/item/11658/diy-heat-mat-speeds-seed-starting email this week but it used expensive rope lighting that I did not have on hand and many of our local stores did not have in stock due to clearance sales. I also was not sure I wanted to construct a box of plywood right now as we are redoing the kitchen and that takes priority. I did have a heavy cardboard box the right size and several strings of mini-lights that are not LED  to make a try. 

1. After cutting the box to size for half my windowsill  I added a 3" wide box in the center for stability and possibly to set transitional plant on top of. 
2. Next the lights were divided between the 2 halves and mostly  positioned under the lids of egg cartons. This give a good surface to set the plant trays and keeps the light a bit subdued. 
3. Next I secured enough plastic to make a cover and added a couple of pieces of styrofoam to insulate near the window. The plastic keeps in the heat and humidity pretty well. 
4. I added a tray of seed for the first trial. The temperature at top of soil level is an even 70 degrees which is perfect for the tomatoes. We used the top of the  freezer last year but some of the seeds need light to germinate or shortly after and the hotter and cooler cycles probably weren't the optimal either.

I  made my own starter cups using a clear XL egg carton with its top as the drip tray. Of course I did use the organic seed starting soil I have used in the past. Next week we'll see how things are actually progressing.