Keith, assuming that you are shooting with an X-T1, then in my experience the ex-pro white batteries last longer than the originals or any others that I have tried.
https://www.amazon.co.uk/Ex-Pro-1260mAh-Replacement-Fujifilm-NP-W126/dp/B00EXP4Z9U
Also in my experience you have to be taking an awful lot of long exposures to actually run out of battery, and to be taking them all from exactly the same position seems unlikely unless you are doing star trails.
I would invest in a plate modifier for your tripod to convert to Arca, buy an Arca compatible grip and then that will allow access to the battery, but I suspect that unless your tripod is super sturdy, and you have really delicate hand movements, that you are going to move the camera position anyway, changing the battery in situ. Possibly better to mark the Arca grip/plate, so you can remove the camera and replace back in the same position.
Unfortunately I can't see any free solutions, they all cost money.