I've read that the Aztecs used blocks of sod to start squash in, then transplanted the seedlings out.
Some folks like using soil blocks, since these seem to lead to higher quality root systems. However, they can be difficult to keep together, and can use a lot of imported ingredients.
I'm wondering, what if we just used blocks of sod? (Dead, of course.)
In fact, what if we grew special sod? What if we laid out a 4 inch layer of compost, soil, wood chips, amendments, whatever mix we fancied, on a plastic sheet, planted a mix of plants, and watered it for a month or so to get it growing? We could then let the plants freeze out if we chose the right ones, or we could cover it with black plastic to smother it. Then we could cut the sod into squares.
We could use all sorts of plants. We would probably want a fast growing annual grass for a lot of binding roots, nitrogen fixers, and any plants that are considered especially stimulating for selling growth. We could even include a few things that would probably survive and be of benefit to the plant once it is installed. Oats would be a good grass, since they can grow well in the autumn but will eventually freeze out.