Coldframe

Excellent suggestion from SpotsOfTime. Now the bricks I have promised my brother are no longer annoying me in a heap on the patio, the old greenhouse door is no longer leaning against the fence, and I've planted most of my seeds.
It's not very airtight, but I think seeds have germinated in such conditions before.
I haven't used potting compost but ditto.

And if it all fails, there's always the supermarket. By the time I know that, my brother will be about to collect the bricks (maybe) and I can Freegle the greenhouse door.

(The bit of ply lying on the soil behind, I have dragged out from under its groundsheet against the wall for my next project. (To be continued...)