Well done for cracking on with this one. I have used http://www.acorncanvas.com to good effect for covers I made for my Broads Cruiser. Dont forget that Orwell Yacht Club have a ‘sail loft’ and machine in their ‘sail loft’ for use by members.
The additional tie-downs will help – a key for covers is to prevent pooling of water on the top.
I still thing your cover is fantastic and am planning to replicate (in two parts – fore and aft).
The other thing word considering is a tape or shockcord around the ‘gunwhale’ perimeter . This is how all those dinghy covers yours is modelled on resist the wind……
Or you could have something slightly more difficult to fold up as in the picture…..