Check. It. Out.
I can't remember the name of the forummer I owe credit to for showing me this type of hem (with a channel), but others will, so thanks, That Guy! I did a hem like this all the way round to give the option of a shock cord that will tuck just underneath my feet and shoulders. I sleep like that on a bed, you see.
The filling was a b*tch but with four hands we minimised the mess. Couldn't find a fifth hand for photos, so we'll skip to the pinning stage of the final side with filled compartments:
I'm leaving it as a rectangle for now to see how that works out. I'll be able to modify a footbox into it on the road if I feel the need:
One or two minor imperfections but nothing that stops the thing doing its job:
I am delighted with my new toy