[Hectic]

^that. Unless you have other batteries to try and fit buying them to find they dont fit is useless.
Other option would be to go lipo.
You can get a low c rating lipo and run it safely but it would be better to also get a baisic mosfet to eliminate arching at the trigger contacts as they will eventually burn out if you use a 20c or higher lipo (at least thats my understanding) tho if you get a nice MERF 3.2 like i just bought you can set up 3 rnd burst and all that cool stuffs.
Most smaller lipos should fit in the rail system no problem (like the buffer tube type lipo or simaler). Other option would be ghetto but you could just tape the battery on so you dont gotta use the grip thing.

Either of the options i posted wouldnt be too cheap.
A merf is 70 bucks plus lipo and charger and rewire to deans say another 70-100 bucks.
A battery box and a battery that fits it will be cheaper.
And tactical tape mod would be cheapest (and fugly but functional)
Maybe at a gake ask some folks to try a mini nunchuk bat to test fit as it may or maynot fit. Tho based on the size of them id say likely not.
Last option that may or maynot work sould be to see if yo7 can fit an AK stick type battery in the body of the gun like how the mp5k is search cyma mp5k battery install on google youll see what i mean.