Here is my frame idea, a day late due to work commitments! The frame is made from tile lats (unsawn pressure treated timber used for roofing). These are available in B & Q, or from builders merchants, timber yards etc. They are screwed together into a rectangle which goes to the outside of the posts.
A large screw is driven downwards into the top of the posts, to prevent the frame from moving around, and left slightly lower than the top of the frame, so as not to tear anything placed on top, like polythene or netting.
And then the protection of choice can be stapled to the frame and weighted with bricks at the bottom.
Will this work? I really don't know. I suspect it will work better with netting than with polythene. This polythene is not very durable (99p a metre from garden centre - annoyingly I know I have squirreled away lots of polythene and couldn't find it.) On reflection, I think bubble wrap would work better. I think that if we get high winds and storms forecast, I will remove this - given it is there to warm the soil and protect against cats and frost, and storms at this time of year will not be combined with low temperatures.