Quick and easy. Pull about 8 inches of core out of double brand. Re run the core back thru the the cover and put a few stiches in. Tie the core to one side the cover to the other.
That's pretty much what I sent him. Difference being I put the cover through the core, and the core through the cover, then there's no need to throw any stitches in. It locks down pretty hard.
What I want to try doing is taking a short piece of dyneema, splicing an eye on each end onto the eye straps on the rear beam, then splicing an eye on the core of the actual sheet with the split tail going through that eye, then just pull the eye splice back into the cover a bit. There needs to be something there to lock the cover to the core on the sheet though, just stitching it would probably work fine. Doing that way should make it self centering, and should run pretty cleanly. Once in a while I get something that hangs up and I have to pump the traveller sheet to get it centered.