Hi,
When i ran a seperate Rx pack i strapped it to the top of the front lipo, that way it was always removed with the main lipos after a flight and able to charge off the heli.
I have seen them strapped to the side of the main frames, under the ESC plate.
I have recently moved onto using a Gryphon Quasar 75v BEC and Scorpion Back-Up Guard, just for simplicity and ease of charging etc...
Hope this helps,
Cheers,
Ross