Heh as you can see I removed the stick battery thing becuase I don't know if the G36K has enough space for that. A stick batt is ~21cm in length, do you have that much space?
Usually when I installed the battery, I put the gun upside down to slide the handguard on. It can be difficult if the gun is right side up because you do have to juggle things a bit. Does your connector get in the way?