The problem with the open sleeve on the back of the T bag is that, if you want to use it for a water bottle, the bottle tends to stick against the fabric of the pocket as it is used. That’s a pain when you are riding.
One of the clever fellows on Brompton Talk (I’m sorry to say that I don’t remember who) said that he’d slipped a cut-off milk carton into the open sleeve at the back of his Brompton T bag in order to solve this difficulty. UK milk cartons must be a better fit; ours here in the USA didn’t work well.
However, a cut-off seltzer water bottle worked perfectly. (Something slightly larger would, too, but I’m not sure what that would be.) Above: back of the T bag, cut off Faygo bottle (midwestern? must have bought this back from a trip), and tall Sigg metal water bottle.
Seltzer bottle dropped into the T bag sleeve. There’s room for a larger inner “sleeve”, but for my purposes, this works just fine.
Water bottle in place, and now super easy to lift in and out. No drag at all — it’s a brilliant, low-impact fix, in fact!
Notice the fuzz on the pocket in that final picture? Fuzz that isn’t present in the first image?
I had “help”.