I decided that simple was best. I went down to the lumber yard and got 4 equally sized pieces of 1×8s – each piece is a hair under 4′. Then, I stopped by Lowes and bought a $2 polyethylene drop cloth, 1 mil thickness. Then, with Meg’s help, I used wood screws to fashion the frame and sealed it using a water-borne acrylic coating. At this point, the frame looked like this:

I let this dry and then used a staple gun to attach two layers of the plastic sheeting. I figured that by doubling it over, I could create a warm pocket of air between the layers that would provide good insulation for the plants beneath. The sheeting was pretty fragile to the staples, so I wound up having to roll up the edges of the polyethylene to get enough material to fasten. That seemed to work pretty well, though, and the result is this:

Now, I need to measure how much heat I can trap underneath it. I’ll borrow a thermocouple from work one day after it gets cooler and measure how warm it gets under there at the end of the day. With any luck, it’ll trap enough heat to keep the germinating seeds happy during the early spring chill.

Update: I added some corner supports to the frame. While I was going for the “cheap, simple hack” version, without the corner supports, it was hard to lug about. See this post for details.