To give credit where its due, I got the heater idea/plan from The Door Garden. Thanks!
This is the bottom. I used a piece of damaged blueboard (drywall) from the local hardware store. It's cheaper that way. The rope light was full retail. They come with clips that screw into the blueboard. You can use tywraps with a screw holder. The bits of wood for support are scraps from old stakes.
I recommend laying out the rope for even distribution. If you use tywraps like I did, leave them loose so you can slide things around.
I didn't use insulation between my rows. The bits of wood are to provide support. I do have wood all around the edges.
I'm wondering if filling the inside with sand would provide some thermal mass and keep the temp more constant.
I got a roll of cheap, ugly vinyl linoleum and glued it to the top of the blueboard. You want to keep water off it.
I wonder if a really thick layer of paint might work. But I drag trays of seedling across it so I'd be concerned with wear. Obviously, I don't care about looks!
I used screws to hold it to the scraps of wood at 4 corners. I might want to remove it to replace the rope or take photos :-)
I notched the end of one heater to fit between the rack legs. On the other one, the wood on the sides sticks out around the legs. That might be easier.
I felt I had spent enough on lighting for one year.
Note the red bungie cord on the lights. It's inside the chain, going from post to post. By sliding it down on top of the lights, it prevents swaying.
The lights are close to the cups. I run them a few inches over the plants as they grow. Moving the plants around can even up the growing.
Hydro Farm digital controllers. They're inexpensive and have a temperature probe to put in the soil of one of your seedlings. The digital lets you see it's set to a specific point. They handle up to 1000 watts. I think both the rope lights I have are under that so I probably could have used one.
I have a power strip tywrapped to the rack. One end goes to a timer, all the lights go in the strip. Lights will be on 12-16 hours per day.
The heater controllers go into another strip.
The whole rack is on wheels. If I had it in the garage, I could wheel the whole thing outside for hardening off. Unfortunately, my basement has two steps down. If I want to put it outside, I have to take it apart :-( It's good that I have those coca-cola crates.