i have the lycaon but the g&g handguard looks different then mine. it looks too narrow on the side closeest to the mag. my 8.4V battery fits(very snug) in the regular mp5 handguard. guarder has some oversized handguards (google this, Guarder MP5 SD RIS System) that could fit a bigger battery, but they look sorta huge.
i made a post about fitting a battery in a mp5 ras handguard, have a look