Propogator covers

Plastic  bags (freezer etc) with lolly sticks to keep them off the soil, or simply inflated prior to being sealed.

Sheet of glass until the seedlings show through.

You can get cheap tops for seed trays at this time of year from garden centres, to make them into propagators too.

I use a bit of cling film over my seed trays until they germinate - not free but very cheap !

If you can get hold of them - those large, plastic water-bottles that are used for drinking-water dispensers would be ideal once cut on half. They would give plenty of headroom for seedlings and most probably cover a 12" tray.

They are not easy to come by but cracked ones are of no use to the water-suppliers.
Just a thought.

I try to ge to the supermarket when they are restocking the fridge section.
The trays that some of the stuff comes in are clear rigid plastic trays and make great covers for seed trays.
Just ask the bloke restocking and hes happy to give away

That's a good idea. I'll keep an eye out next week when I push the trolley round and while I wait for TOH to finish nattering to friends...