I would also advise against using a transmitter battery pack.

My favorite battery for the Slow Stick is the 8-cell, 5/4AAA, 800mAh, NiMH pack used on the Crazy Max from Wattage at Hobby People. It costs $33 and is often on sale. The pack weighs only 4.1oz with connector.

You can also buy them at Batteries America.

Good luck!