I've had my JBC iron for about 4 years now, and it lacks the fancy built-in storage for C245 cartridges like the new models do.
This is my V2 of the 3D print I have been using to store extra cartridges for about 2 years now. This version has tighter tolerances and supports more of the body than before, leading to a firm holding.
The ID of the cartridge orifices has a +0.30mm shrinkage allowance built in for PolyMax PETG, which in my case was calibrated specifically for my printer (non-stock MK3S). I have included the design files specifically for the purpose if you find the holes too tight/loose. You will need to figure that out for yourself if required.
Mounting: Double-sided tape, suggested 3M as the knockoff stuff is pretty weak. I went this route so you can mount it anywhere.
The author hasn't provided the model origin yet.