Here's my half-arsed-makeshift-no cutting-no sewing-no hassle attempt
I haven't tested it in the woods but I whipped something up using an old bag unzipped and whoopies to tie it to the ends (If I don't end up with a real UQ I will likely swap out to shock cord). I also need to do something to gather the head end but still being warm, it isn't a high priority. The way I last tried it the bag is slightly twisted to the non-entry side to allow me to get in and out as well as for the footbox but this means the side tie out on that side is blocked. Seems pretty warm. I am sure that the bag should be snugged up in parts but that can wait.
Took less than 5 minutes to do.