I first came across the idea of a fabric pocket attached to the backplate in a Halcyon catalogue. I liked the idea of keeping my backup SMB in it, as it occupied most of my drysuit pocket.
But it wasn't quite what I wanted, so I made my own.
It differs from the commercial ones:
But of course, it's just like the commercial ones, in that I store my SMB in it...
The purple fabric is Cordura, very tough stuff. The velcro is industrial-strength stuff, held on by fishing line.
It's held on with a loop of reflective bungee, which loops through the backplate holes and the pocket holes and back over the plastic toggles:
