Woodcraft (and some other vendors too) have a new 3/4" trimmer with multiple shafts. Should handle most kits.
The only problems (to me at least) is that the shafts are NOT threaded. Seems to be designed for chucking into a drill press for use. The shaft end (for chucking) is too short by my standards. Not confident that the mill will rotate true when chucked into a drill press. For this reason, I prefer to square-up hand. I have some older shafts with threads that I carved handles for hand use.
On the subject of the various tubes sizes for all the different kits, I turned sleeves for all the tubes out of some spare white corian I had laying around. Wood sleeves are too brittle for thin sleeves, even when using a 7mm shaft. I also make them longer that the metal sleeves you can buy. I have a set of micrometers for checking that I am getting the sleeve down to the correct thickness. When close, I stop using a tool and sand down to the final dimension. I then mark the appropriate dimension on each sleeve with a Sharpie marker (i.e. 15mm, 31/64, etc.). I have a box with a all the dimensions for the pens kits I turn on a regular basis. If a new tube comes out, I turn an additional sleeve for the collection.
If you trim by hand, the sleeve is mandatory. Using a mill that is not snug in the tube allows the mill to wobble and you do not get an exact 90degree trim on the blank, and it will be noticeable to the naked eye. If you chuck the mill in a drill press, then you shouldn't have any wobble, and you 'may' be able to forego the sleeve. I would test this before trying on a live blank. You can chuck the mill in the press, scrap piece of lumber under the mill and lower the mill until the shaft touches the scrap lumber. Rotate the chcuk and mill by hand and watch whether the shaft tip wobbles out of true. If it does, then use a sleeve.
Some people sand the blanks square, but you have to either have the bore exactly perpendicular to the side you lay on the sanding table or you need a jig to hold the blank perpendicular to the sand surface. Mostly of the jigs I have seen use a mandrel mechanism similar to the lathe setup. For that, you would then need a sleeve to fit on the mandrel to keep the blank from wobbling on the sanding jig mandrel. Same problem has a hand driven pen mill.
Several means to accomplish the same goal. Pick the one you are most comfortable with and run with it. All have their own advantages and disadvantages.
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