hopefully someone with more battery / electrical knowledge than me can answer this!

the battery in a joystick is an 18650, and the joystick comes with a lead thats usb on one end and joystick smart port on the other.

would you be able to use a power bank type thing instead of an exposure support cell to give additional run time? The Anker power bank things on amazon start about £10 for a 3100mAh ish lipstick type one.

would this work? Or can you get power banks that you just put an 18650 battery in?

Yep I think that should work. You’ll have to check how much power the Joystick draws and how much power the battery bank is capable of providing.

Eg if the Joystick requires 5 watts on full power then the USB battery bank needs to be able to provide the same output too.

Otherwise best situation is you don’t get full power from the light worst situation is you could end up damaging the power bank by trying to draw more power than it can provide but the Anker products tend to be reasonably well designed and should have protection circuitry to prevent damage to the electronics or cause overheating and fires.

And yes there are battery cases where you can drop your own 18650 battery inside. If you already have batteries and chargers this is probably the best option as the case won’t be smart and will allow the the Joystick to draw all the power it needs. just need to solder on a cable and plug that is correct for the Joystick.