Last week I finished my small cold frame for the balcony.
It took very little extra work, in fact: I just had to cut an appropriately-sized chunk off one of the 2m2 pieces of polycarbonate I brought home on my bike trailer a fortnight ago:
(Another demonstration of the truth of my long-held belief that you can transport pretty much anything with a bike. Six miles -- albeit quite slow ones -- from Dulwich to Bermondsey and I didn't have to stop and retie it even once. I was very pleased. NB: the polycarb wasn't touching the ground at the end when the trailer (a Carry Freedom Large -- fantastic piece of kit) was properly attached to the bike.)
The jigsaw went through the polycarb with no bother at all, and I taped the edges up with gaffer tape. To get some air into the frame, I'm using part of one of the planks I cut up for the slanted top: the polycarb lid just rests on it at the back. I haven't bothered to make hinges; I'll rethink that if the lid doesn't stay put.
Next plans: slightly bigger cold-frame for the table of herbs outside on the balcony, and much bigger one for the allotment. Currently in the allotment there are three rows of various sorts of greens under mini-cloches (cut the top off a one-litre juice bottle), so the cold frame needs to be built before they outgrow the cloches.