The builder asked me if I wanted to keep the them. I didn't particularly, but if he took them off to be broken up for scrap wood he would have to pay whereas if I did it I wouldn't. I was about to shove them in the van when I had a blinding inspiration, grabbed a couple of bits of chip board, some screws and some wood preservative and knocked this up:

Now I have a proper place to start my seeds off, just in time for the order to arrive.
I've still got one pane left....