Sounds like a battery extender is all you really need. I have seen guys use short pieces of copper pipe with caps on the ends to finish off the connections and using some tape or shrink wrap to keep the copper tube from hitting the flashlight tube. Then maybe using some plastic spacers on the 18650 to make up for the larger diameter of the C cell.
Or maybe run it with 3 Eneloops with C sized spacers they use. That would work I think.
I don’t think you would have to change the driver as long as you are happy with it. The voltage of 3 C cells in series vs 1 18650 is so close that it won’t matter to the driver I wouldn’t think. The driver doesn’t know the difference between LiIon and Alkaline as long as it sees it’s DC voltage tolerance.
Any light that will run on 3 alkalines should run just fine on 1 lithium ion cell with no change in driver. All that’s necessary is using a spacer so that the battery or batteries can physically make contact with the ends of the battery tube.