Inokatsu Galil ARM Battery issues

[Jockanease]

Looking for advice on what battery to fit in my Galil?
Have been using my Ak Lipo (7.4v,1400mAh) to test the gun and would like to get the right one for the job.

Battery space is similar to AK with the addition of the gas tube which is 15mm diameter int. and 200mm length.
Would like to keep using the Lipo's but havent found one to fit yet, the gun will be used as support so its going to need plenty of juice!!

Cheers