onboard guitar buffer power options

[claytushaywood]

anyone have any ideas for powering an onboard buffer besides a 9v battery?  I was thinking maybe some watch batteries or a lithium battery- really the goal would be to be compact or flatter than a 9v so i can fit it into my strat that will have some other mods.

i see fishman has a rechargeable lithium battery pack for their fluence pickups that replaces the tremolo cover on a strat- pretty cool... but $100