A.R.M.S - 3xAA battery clip for Mk 1 shooter
Here is a 3xAA battery pack holder to suit the Mk.1 ARMS shooter (http://www.thingiverse.com/thing:454862), or other models using the same mounting hole pattern.
Battery clips to suit this holder are available on eBay.
Note: Image shows handle as well as clip, just to illustrate how it is to be installed.