So, we rearranged everything in the shed. We removed the round brooder, DH rigged up some "sides" using various leftover bits of Cube run, and the dismantled run from the Go. We had some spare matting, and so the chicks inside area has now more than doubled. At the same time, we switched the feeders so they are now using the oldest style Grubs. These are difficult for them to poo in: even if they stand on top, the poo can't get into the food.
They seem quite happy for now. Hopefully tomorrow will be drier, and we can let them out again.