Dan, how much longer is the outer cap of the XL over than the normal version? I'm thinking maybe they can both just use the XL outer cap?
Here's another idea. Instead of the XL outer tube, we'll have a threaded ring as a spacer for the normal version to extend it longer. I got the idea from the Provari Power 2500 battery spacer.
For example, if the XL outer tube is 0.393" longer, we'll have a threaded spacer for 0.393" long. So, to use the bigger 4.5ml XL tank, we'll screw on the spacer first and then screw on the normal version outer tube and it will be extended to an XL. I'm not sure if it means that we'll need more threading. What do you think?