I had this in my (standard style handguard) G&G G36C, and a variety of my other guns too.
https://hobbyking.com/en_us/turnigy-nano-tech-1200mah-2s-15-30c-lipo-airsoft-pack-t-connector.html
It's very compact, fits in other RIFs which don't have much space, and it's been reliable.
And it's dirt cheap!
Assuming your G36 has a MOSFET so it can run 11.1v batteries, I recommend buying a couple of these.