Component Shop def will do a standard 7.2v battery, had one made for mine, nice to see someone else with a Marushin Uzi! Just email them a pic. Think it was about £16 a couple of years ago. Wouldn't mind another couple of mags and a piston if you ever sell any lol!
edit its a 4 over 3 cell with the cells arranged across the gun parallel to each other. Make sure you specify long leads!
i would be interested to know what response you get, only i have purchased an Amoeba CCC and that has a very small but long battery compartment (7.4 Lipo 1000mah nunchuck fits but that is about it). i asked Componentshop re modifying a lipo and they said no, but didnt ask about a custom made one.