On a recent foraging trip, beneath the brambles I found a rusting umbrella covered with clear plastic. It was almost undamaged and on opening it out I realised it would make an excellent protective cloche for my overwintering salads.
I removed the handle to leave a long spike which acts as an effective ground anchor.
If necessary some ventilation holes could be made in the plastic at the top. And of course, when not in use, it folds away very neatly.