Well, we got sorted in the end, and
@Woody girl has a rucksack belt that fits.
I used waterproofed polyester canvas (heavy duty stuff) and webbing (1-inch and 2-inch). (I tend to keep a stock of both items as they are so useful for all sorts of stuff). Plus a roll of 3mm sticky back foam stuff, (3 layers) and a Prym 2-inch buckle (Prym is a decent quality supplier of notions and tools for dressmaking).
Here's a gratuitous photo of the attachment method:
Those little turnbuckles are easy to get, inexpensive and they make tensioning the webbing across the frame really easy. But failing that, it could be laced on with paracord.
The belt webbing is attached to the webbing which wraps around the frame so the load is all taken on the webbing. I used 2 one inch straps as that allowed one above the shelf and one below to locate correctly and keep the belt in place. Then there's padding inserted and of course the padding is all wrapped up in canvas:
My sewing machine is an "ordinary" domestic one (not even a Quilter) and so it was a bit grumpy about sewing some of the thicker areas, there are a couple of snaggles where it threw a wobbler on me. But it did cope.
The order of construction is important: Frame webbing loops made, then canvas onto those straps, then belt webbing strap attached, then finally the foam into place and sew it in, then tuck in the corners. No pattern- I just started with big pieces and cut/shaped as I went along.
@Siberian Mongoose - of the frames I have played with, the old Karrimor and Berghaus ones seem the best made and most comfortable.
What strikes me with this little project is how adaptable and versatile these old frames are. It's good to bring them back to life, next idea is try is making a decent shoulder harness. I have a bare frame (the one I used as the model in this) so making a full set of fittings for it (shoulder harness, belt, bag) would be a good project.
GC