Inspired I had a go. I wanted a small panel hoder for 18x24 cm panels as I like the Belle Arti ones that Jacksons sell - relatively cheaply in bulk. most panel boxes are in imperial size . I had a sheet of corrugated plastic left over from some I bought about 12 years ago to make a secure folder I needed for some very large (5 foot by 3 foot) photographs I had printed.
I was also looking for some suitable plastic channel and found this in B&Q a DIY store chain. It cost around $10, but now I have enough to make several boxes. The other materials needed are a small saw, hobby knife and duct tape as well as some contact adhesive.
cut out, scored and channels glued in - (nearly got it right)
folded and taped - inside view
rather clunky completed box, but small and light enough to fit in my day pack
many thanks to all who contributed to this thread