Summary

This is the back section of the SRM PCV case which will hold an 18650 lithium ion battery. I desoldered the original connector from the pcb and left it in the original back, just in case I ever want to put it back to original. The 5pin binder connections are available from the likes of RS, I epoxied a new one into the new back to ensure it's still rain proof, the original rubber seal still fits this.

I also epoxied a couple of metal battery terminals to each end of the battery compartment so that the battery just slots in place and is easy to change.
I disconnected the charger wire from the external connector as I'll be charging these on a bench charger and just replacing the battery when it gets low.

The model isn't perfect, but it works fine for my use.