No need to decase your Gopro! You can save 20 gram by simply powering your action cam over its USB port. You only need breadboard jumper wires, a USB step down module and an USB-C plug with PCB. My 3inch cinewhoop weighs only 236 gram with a Gopro Hero 7 black and a Tattu 4s 450mAh battery AUW! In this video it is shown how it's done!
http://on.fb.me/unboxingexperience