My post about the missing milk cartons got some attention. Here's a photo of the salvage operation in progress.
I think I made the seedling flats sound complicated. Doesn't everybody use milk cartons this way?
The two at the rear are ready to go, except for dividers. The rear one has its cutout panel sitting in it, down the middle. To make that panel into a divider, I just cut three slits in it, halfway across, and then added some cross-wise pieces. Each cross piece is the same size as a milk carton base, and has one slit cut in it, halfway across. Put the cross pieces onto the central panel, slit to slit and then press them down to sit level (hard to describe but very easy to do - you probably made little stand-up paper dolls or Christmas trees this way when you were a kid). Square them up as you fill the cells with soil, and this is what you get:
I think the cells may be a little bigger than necessary, which could be a problem if I can't come up with enough space in the sun. Right now the flats are just sitting on top of the fridge - theoretically the warmest place in the house, but with all our energy conservation efforts, even that spot isn't terribly warm. Good luck, little seeds.