More of a hack/repurposing than MYOG, but I managed to add two pretty big hip belt pockets to my Miniposa for $14 and 1.2 ounces.
I was lucky enough to get a Gossamer Gear Miniposa from a fellow BPLer, and I'm pretty excited about it. I'm coming from an Osprey Atmos 50, which carries with amazing comfort, but weighs nearly 3 pounds...but I completely adore the long hipbelt pockets on the Atmos, which the Miniposa (and the Mariposa) lacks. I wanted to add some, but I don't know how to sew, I didn't really want to do any permanent mods (yet), and the Simblissity UnSlack Pack didn't really fit the bill (overly complex).
Somewhere online I saw a picture of someone using one of the lower rear tieout loops to string some cord along the hipbelt from which to hang things (sorry, can't find the link now)...the lightbulb went off and I went to REI. I searched around and finally spotted the Eagle Creek Pack-It Sak in extra-small. They are made of mesh with a transparent anti-spill coating inside. It exactly matched the length of the Miniposa's size medium hipbelt wings. I bought two for US$7.00 each.
All I had to do to prepare them was to razor off the tags and hacksaw through the hard plastic clip. After this, each one weighed 0.6 ounces (17g). I'd estimate that each can hold about 75 cubic inches of stuff.
I set up some rigging on each wing of the hipbelt using BMW AirCore 2 accessory cord, between the lowest rear tieout grosgrain loop and the start of the hip belt's webbing, and strung one of the Pack-It Saks on each side. It took a couple of times to get the rigging secure so the pockets wouldn't slide and wouldn't pinch the front belt webbing. I even tucked the rigging into the hook-and-loop closures for the hip belt padding pockets to make them even more secure. As long as the Spectra cord doesn't fray, they look good to go. The photo below shows them attached, each pocket holding three energy bars with a bit of room left over, which is certainly the most weight I'd possibly put in them.
The same thing could easily be done with some self-adesive velcro towards the front of the pouch and a ziptie, Jason Klass style, between the rear pouch loop and the Miniposa's rear tieout loops. I may attempt such a slightly more permanent approach if this hack works well in the field (trip coming up next week).