You could use calipers and mic the size of the hardware. Either turn between centers or use some 7mm bushings as spacers on your mandrel. Then mic each piece of hardware and turn the matching end of the blank to that size.
If the tubes of the pen kits are larger than your mandrel, you could make wooden spacers. Again, turn some scrap hardwood on the mandrel, maybe go ahead and glue in 7mm tubes. Turn to size so they will fit inside the kit tubes and act as your spacer. Probably a good idea to wax the surface of wooden spacers before inserting them in the tubes, especially if you plan to use CA finish on the pens...