Solder wires from the battery clips of the most reliable flash to the external battery case. This flash will be the trigger flash. Hot glue the flash into position in cookie tin. Solder the trigger wires from the flash to the phone jack. Plug in the RD616 wireless trigger into the jack and test. Hot glue velcro to the outside of the cookie tin to hold the RD616 in place.
Hot glue the slave near the trigger flash. Solder the second flash to the slave and the battery pack and test. Do the same with the third and fourth flash.