The backpack print from Shapeways has arrived!
Overall I am very pleased with the quality of the print. The walls are only 1.22 mm thick which makes the design somewhat delicate with the standard plastic material, so if I had to print it again I would thicken them. The dimensions perfectly match the screws, battery and back of robot chest, so the backpack fits in without any problems. I should have also added a few more holes around the sides, as the wiring is fairly cramped. The battery cabling is fairly awkward, maybe I should have gone with an even smaller one than originally planned!
Along with the battery, the backpack also houses the SMPS2Dynamixel servo power adaptor and a bunch of wiring. The power adaptor has a tricky shape with servo connectors at a right angle to the DC jack, but I was able to use a smaller plug with its plastic casing removed in order to fit it inside the limited space.
Hopefully the battery is close enough to the centre of mass, and not too high, so it remains to be seen if the servos are powerful enough to keep the robot standing and moving upright. Now it is back to software, to get the robot to finally do some interesting moves!