I use these...
https://www.vapextech.co.uk/airsoft-7-4v-850mah-30c-lipo-battery-with-case-lp007-vapextech
I needed a small battery for my L85A3 so bought 5 of these, I use them in pretty much everything except my AK's & AR15's as they mostly need a narrow stick or nunchuck style. They're ideal for my Honeybadger due to the short butt stock.