In another thread I mused about getting a Condor T&T Pouch for my Brain Bag, and whether it'd fit with the annex clips. I decided to buy one (in black of course), and WOW. All I can say is, go get one right now.
The Condor T&T pouch is an "admin" pouch for the prepper/pseudomilitary set and also EDC folks. It has a zillion pockets and straps inside for pencils and knives and flash drives and whatnot. Here's a video showing how the pouch works (the first thing I did was remove the map cover and the pouch strings). Downsides: the stitching is poor. The bag is fairly heavy (400 grams, almost a pound). The elastic inside, I am told, wears out fast. The zippers aren't YKK. But did I mention it's only $20? And a perfect fit.
Below I show a variety of ways to hang the pouch in your Brain Bag. In all these pictures the pouch is empty. So first let me show a picture from Sid O'Neill which has the pouch attached to his Smart Alec and stuffed to the gills. It's rather bigger looking in the Smart Alec than in the Brain Bag.