I guess you'll have seen these already? https://www.thebushcraftstore.co.uk/survival-metrics-resqbrite-signal-panel-16488-p.asp
They're out of stock right now, but I put my name down to be notified a while ago, and have bought one as a result. I think they are pretty good; a bit pricey admittedly.
There's also the Prepared Pathfinder guy on YouTube. On a recent video he shows a very simple tube of reflective material that he loops over the top of his rucksack. He says he had it made up, and I guess as it's basically one line of stitching, one option might be to get some material yourself, then take it down to your local repair person, perhaps at a dry cleaners, to get it made. It would probably only take them 5 minutes. Just a couple of thoughts; hope they help.
Is the purpose to keep warm and visible in an emergency or just Visible in case of friendly fire or during the UK moose hunting season?
I carry this and similar for emergency use: https://www.blizzardsurvival.com/shop/blizzard-2-layer-active-range-survival-jacket/
Can you sew? You can buy the material and make what you want or pay someone with a machine too. The reflective material will be sweaty and non breathable next to the skin.
Might be worth sticking your name down to be notified anyway. As I say, that's what I did and they do come in. Or maybe give the shop a ring?I had , but as you said out of stock - Its a shame because its around the sort of size of I'm looking for - I thought it would be good to see if a local maker would be interested in being commissioned on one.
You beauty !!!How soon do you want one?
I could do a couple for you for about £20 which is the cost of materials + p£p but wouldn't be able to for a month or so.