Here's how I installed a 14.8 volt Li-Ion battery pack (also from Batteryspace.com - the big green thing on the left side,) a 3-inch speaker, an AirWire receiver/decoder, a Phoenix P-5 sound card, volume toggle switch and a programming jack inside a B'mann Mogul tender. Also, a recharging jack and on-off switch are hidden in the water hatch.
The coal pile is removable to allow access to the stuff in the tender. I found that because the tender is made entirely of plastic, I could leave the antenna laying flat inside without loss of range.
Here's what the battery pack looks like (below.) It's a Li-ion 18650 Battery: 14.8V 2200mah Li-Ion Battery Module ( Long ) with PCB from
BatterySpace.com (click for a link to their web site.) Cost: about 65 bucks.
As I mentioned above, the heavy battery pack had to be set to one side of the speaker magnet in order to fit inside the tender, but that gave it a severe list to port so I had to add lead weights on the opposite side to counterbalance the tender on its bolsters.
As the old saying goes (with apologies to my fellow cat-lovers,) "There's more than one way to skin a cat." This is just the way I did mine.