I had a similar idea of sci-fi tubular bracer a while ago, but I was inspired by Lila from Futurama I even molded it of clay in order to understand the optimum volume and geometry necessary to integrate some of my electronics and allow it to open/close/rotate/stay in place easily. That's a lot of space if you don't mind your forearm looking like 1"1/8 thicker than normal I was able to integrate my PDA, mobile phone, GPS, Keyboard, and 20 AA cells easily along with wires, in- and out- connectors, lids, locks, handles and even a couple of ventilation fans in each half of it It was still like 1/4 of volume available for more ideas. An that's not like just a storage for all that stuff, but rather a single integrated device. The only problem was that it's too individual in shape. So, something like cold liquid plastic sheeting will be required to mold the shape of the inner surface right in place. And yes, it will be quite heavy.