Have to say I read about this somewhere on here a while ago and it works brilliantly. We bought three of the Tarps from Decathlon - £20 each including loads of guy ropes, pegs and two five section poles. They are a great size, good lightweight but strong material and can be used in so many ways.
When thinking of using something flexible but strong for the insert suddenly came up with 3 core round electrical cable of the right diameter - works great! My wife hemmed the tarp over a couple of inches first then we hemmed in the cable length really tight by first using a smaller diameter wooden dowl rod as a guide and then slowly fitting in the cable - feeds in the Cali awning easily.
You can then rig it up as an extension to the ground or flat instead tieing off to trees. A second can be secured with eyelets and elastics to the awning support arms to enclose the space.
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