Maybe you could use milk jugs/cartons to cover them until they get big and hardened?

Cloches can me made from dozens of things. The croissant blister packs from the grocery. Big plastic jugs of nuts from the grocery. Recycled water jugs from your local water delivery company. Pop bottles. Milk jugs. You can purchase cloches, hot caps, lids, etc from a decent nursery (likely not your Bigbox). Mel's old book describes several ways to build wire structures to cover a "square" - I bet his website does as well, but you can use chicken wire, remesh, cattle panel, hardware cloth, poly irrigation pipe, PVC, conduit...

Dan

When I harden off seedlings, the first place they spend time is on my covered back porch. They will get light, but not a lot of sun, and they are sheltered from the breeze a bit. Do you have some place similar where you could leave your flats of seedlings during the day?

I see Susan's answer and rereading the OP I can't tell what the question is. So I'll try this:

Harden off before you plant. This is how it is done any way you garden.

Dan

If they are so tiny, I wouldn't plant them in the garden yet. I'd pot them up into styrofoam coffee cups and let the grow bigger before I let them face the riggors of the garden. That way, you can put them on trays and move them in and out to protect them and locate them in an area that has a wind break and appropriate levels of sun.

Then when they are larger and hardended off, they will be ready to take on the garden world.

Hope that helps!
Susan

Well, I tried just putting them out there, and the tiniest ones just aren't making it. Oh well, better luck next year! I did harden them off on my porch first, but I think rabbits actually ate some of them, and the others just weren't strong enough. I'd started them back in late Feb, but they just never grew past starter leaves, so just I don't think it was going to happen. Some of the larger sprouts that did have good leaves are still alive, but they're yellowing. Not sure why...

Mainly though, my question was simply how to shade 1 square. Mel's book suggests shading cool weather crops to help them last into summer (ie lettuce). I would like to try that but I only have a few squares of those plants, surrounded by what will be warm weather crops. My current idea is to stretch some trellis netting horizontally across the beds and then attach some weed cloth or black plastic or something to the netting just above the square that needs shade. Then some of my other plants (corn, etc) can use the netting for support as well. 2 birds, 1 stone and all that. Do you think it'll work?