Hope I'm not getting off topic (again),   but , has anyone tried using a homemade Tyvek tarp? They're light and strong (and cheap) and present a problem only in the strenghth they might lack at the tie down points.  I think twisting the corners to become 'cord like' and then bending the corner cord around to tie a double loop sheep shank with,makes a pretty strong attachment while the intermitent attachment points could be done with a 'pebble and loop' cord attachment.