I have seen a (nonTB) product mentioned - i'm fairly sure it was on these forums - which was a velcro (loop part) panel that could be configured with various pockets etc - it was not rectangular but wider at the bottom than the top.
I thought I'd noted it down but my search of my own notes and here has been fruitless.
vague I know but perhaps someone remembers such a thing mentioned in a discussion of using additional organising options inside a TB backpack - one apparently fits the synapse(19) and the other was larger .... perhaps one of the EDCForums people mentioned it ....
it's driving me crazy but now that I've put the question out there I can stop trying to remember and get some sleep! (first world problems .... )