I have all the materials to start on this. 8.5' long x 5' wide will allow it to close on top.

M50 and 5 apex.

What I have been wrestling with is the top closure. I want it so that I can bail out if I need to without fiddling with a zipper.

Decided to do an Eno type closure with two cords with hooks only one side will be regular cord and one side will be stretch. The hooks will be on the stretch side.

Like this...

http://www.youtube.com/watch?v=3jpmeQAs5hs

As set up like the Eno fastened on the edge it will not completely close which is fine at some times of the year, but I have decided to have a 2nd run of cord on the outside about 8" from the edge. That should give me a 8" flap and hopefully close the top completely, then I will have velcro tabs on the inside to hold the flap up.

Any suggestions welcome if you have a better idea.