Thanks Martin. I'll follow up your suggestion of pig wire. Yes, my idea was to have a cylinder which would be rigid enough to be fairly self-supporting, but helped by a stake or two. It would be mainly for squashes and cucumbers and possibly tomatoes, although they might be better served by some kind of metal framework to support rows of canes.
Until now I've had a kind of joyful sprawl of tomatoes, cucumbers and squashes in my poly tunnel by the end of the season. The curved framework doesn't lend itself to tying tomato canes in and so they gradually lean in various directions and the squashes and cucumbers simply spread without supports. I'm hankering after a little more discipline this coming year.
Janet G