Inspired by Eebel's battery carrier for Mr. Baddeley's 3D Printed R2-D2 body, this is a battery holder designed to fit a pair of Duracell Ultra 12V 20AH Deep Cycle AGM SLA Battery with NB Terminals (Model Number WKDC12-20NB):
The bottom of the holder is sliced to fit over the splines of the center foot support and can use four of the M4 bolts to secure the holder to the center foot support. There is a loop hole on each side of the holder for a single 2" wide Velcro strap to secure the batteries.
I didn't include a handle on the battery holder because the holder is too large to lift out of the droid without removing the slip ring mount and dome drive gear. My intention is to keep the holder in the droid and remove the batteries one at a time.
A v2 update was added on 22 July 2018, to include cutouts for the center foot screws (8mm around and 5mm tall) in the bottom of the battery box.
Note: The weight of the batteries may put a lot of stress on the center foot support. The amount of stress may vary depending on the material you print with (I only use ABS). I'm considering a mod to the support to make it thicker as well as planning to NOT store my batteries in the droid.