28mm bass speaker fits! If you're using the standard crystal holders from Rudy, you have to loosen the retention screw a bit so that you can shift the collar holding the crystal closer to the center of the chamber. I imagine it would be the same for the Shtok CC inserts, but I don't have them on hand to test.My MK1 currently has a Goth 3D chassis that fits a 21700 and I have a Shtok chassis on order that should fit the 18650. Once it arrives, I'll do another photo test.
Yes, the diameters of the 18650, 18500, and 18350 are the same. The first two digits are the diameter in mm and the next two (plus the extra zero) are the length in mm. So an 18650 is 18mm wide by 65mm long.
Ethan Zimmer posted a video of an 18650 battery in a Shtok chassis that fit (but he's also a bit of a wizard and I may have missed something clever that he did). I think the major change is in the direction of the speaker - before, the speakers had a separate printed holder that had to fit inside the hollow clamp spacer with the speaker facing towards the emitter. With the new design by Rudy, the speaker faces the pommel without the need for an extra holder and all of the space inside the hollow clamp spacer can be filled with a longer battery.