Do you mean polycarbonate (PC) roofing sheets? The corrugated ones are generally rigid and brittle but can be shaped using a hairdryer to heat them - I don't know how they'd fare on a windy allotment site - they act very much like sails so if you were to create a tunnel from them they'd have to be closed at the ends somehow and very securely attached to the ground. I think it would be safer to try and construct something square based around a frame and the kind of polycarbonate that they use in plastic greenhouse panes.
