I was quite happy with the results, so thought I'd share.
From a cardboard stencil of his frame I cut the 2 sides of fabric and a long strip which would make the 7cm width.

I sewed it all inside out, turned and double stitched the seams. Lastly, I sewed in the waterpoof zip from Pennine Outdoors and added cover for the zip end. Fitting the zip proved the hardest but this might have been due to the fact I was hungover as hell when I done this bit. :?

Open up the zip and turn it "outside in" and done.

Obviously its not Wilcat or Apkit quality but it cost about £6 worth of materials and 1 days work.