Hello! Check out the RHINOSKIN aluminum hard case.
Link:
http://www.saunders-usa.com/rhinoski...d=34&model=192
I have an x51v with the Dell 2200mah extended battery and the RHINOSKIN case fits very well and really protects the Axim.
I've had MUGEN extended batteries before (for an Axim x30) and they were the same size (or slightly smaller) than the comporable Dell battery.
...Mugen are excellent batteries by the way!
You may luck-out and find the the Mugen battery is the same size as the Dell extended battery?
...it may be worth calling or emailing them before you buy
IF necessary (you may not have to), the RHINOSKIN aluminum hard case has a little padding behind the battery area. You may be able to remove a little of the padding behind the battery and give yourself an extra 1-2mm of clearance for an even larger battery??
Cheers & Welcome!