As Jo says, the unprotected cells will generally fit, but if you choose to use one keep an eye on the voltage as the Jet-III doesn't have a low voltage warning. One AW Protected 18650 that I have fits fine, the other is a bit snug. This is due to how the protection circuitry is fitted into the cell, if the tab is a bit wide on the end it catches on the side of the tube. Loading the cell from the front of the tube instead of from the rear can also make a difference in some cases, as the batteries tend to be slightly wider at the positive end. Don't force it if it is snug, as that will result in the shrink wrap on the battery wearing through and shorting the PCB protection.
You've got about 18.54 +- 0.02mm of clearance in the tube, so if the battery is wider than that at any point in its length it won't fit.
I have some Tenergy cells that I have tried that do not fit, however others have reported success with them.
From feedback I've seen, more AW's fit than won't. I'm thinking about special ordering some AW cells to see if I can get him to check the diameter of the positive end prior to shipping, that way I'd have a guaranteed fit in a protected cell.
Regards,
Eric