In order to make the G305 as light as possible you gotta use the lightest battery you can.
Does require removal of stock battery tray which includes the dpi button, but dpi can be changed through the mouse software, plus that's just more weight savings.
Designed to screw into the back hole of the PCB, but could probably fit elsewhere. Might require some trimming of the frame, not sure as I've already trimmed it substantially to save weight. Plus makes changing the battery easier.
Updated with slots to hold the contacts in place. May require bending positive contact to reach past slot rail, I didn't need to but it's close to not touching the battery.
The author marked this model as their own original creation.