Disclaimer: I am by no means a designer, I just decided to solve a problem by learning how to make a crude object.
Designed around the Mophie Powerstation.
These mount to the back of my helmet with Velcro, and I use this case to get power to the camera while still protecting it from water/dirt/mud.
The battery pack is a VERY snug fit, and nearly impossible to remove once inserted.This was by design.
I designed this early this year, have 2 and works great. Printed in PETG.
Note: If you choose to print this, keep in mind every helmet is different, and it may not contour well to yours. Because I'm using Velcro to attach it, a perfect fit isn't necessary as long as it holds well. I am able to lift the helmet up by the battery pack without it separating.
I've also included the Sketchup file to edit yourself.
Print with the closed end down without supports.
I recommend stopping the print about 10-15MM in to test measure your battery pack, as your printers calibration may vary.