I designed replacement fuel tanks that fit a common laptop battery (18650) wired in series to get about 7.2v.

It can be a bit tricky to get the wire to fit/pulled through the holes, but it does work. I soldered the wire directly to the ends of the battery but as a warning be very careful and quickly solder so you don't over heat the battery. Also, try not to use bare wire after the solder joint on the positive side, as it could touch a bare spot on the battery case and cause a short.

It prints in 2 pieces, the main tank and an endcap. Once you get the wires pulled through and verified good solder joints/correctly wired in series (see wiring pictures), press fit or glue the end cap on. It'll need a little bit of finish work to fully hide the seam.

One note, after installing the new tanks, I realized the notches at the top of the original tank brackets actually are there to help lock in the spare tire carrier. Mine don't quite line up and need a slight design modification which I may do if these ever fail, until then these are printed. :)

I haven't painted mine yet and just used white filament. Nothing special with printing, though I ended up using tree supports touching the build plate only due to slight overhangs.