cteaglesc,
I'm by no means trying to convince you that an adjustable mandrel would be better for you ;-) There are other ways to turn odd length barrels - e.g. a split mandrel. I think single-barrel mandrels have pretty much disappeared. I bought one of the early adj. mandrels, when only WC had them, and it was extremely poor and non-reproducible. The machining was terrible, the morse taper didn't even fit. I hope they have improved since then!
I don't know if you ever came across the large diameter rubber collars that converted keyed drill chucks to key-less operation. With the large diameter you could apply a fair amount of torque. Something like that instead of the rear wrench flats would be a small improvement. I use a Beall collet chuck, and hardly ever use the wrench - the nut diameter is large and grappable enough I can simply hand tighten. An improved adjustable mandrel doesn't have to cost a lot! A collet for a single diameter needs only a slot - doesn't have to be the multi-slot arrangement that ER series collets have. These collets have to remain concentric over a grip range of 1 mm. If you have a collet that has to grip 1 fixed diameter this is not necessary.
Anyway, it doesn't HAVE to be expensive. Unfortunately, it looks like there is 1 source that all these suppliers buy from... no development on their own...