Can someone please point me to the thread of how people have altered their knitpicks option needles to create a 16" circular? I can’t find it anywhere and was sure I saw it on here before…
TIA 
I don’t think you can do 16", but you can do 20".
I took some fly line about the same diameter and glued it in. Their cables come out fairly easy.
This way I have their cables to go back to if I ever want to.
You won’t have as smooth of a build up as they do but you can get the glue to mold like that a little.
The needles are too long to shorten the cable for a 16 inch. You’d be better off buying the 16 in fixed circulars and saving the longer cable for something else. 
They are only $5 each and surely you need a book or some yarn to go with it. 
http://www.knitpicks.com/Options+Nickel+Plated+Fixed+Circular+Knitting+Needles_NDKPFixedCables.html
Someone, I think Mike, shortened one to 20" which is about as short as you’d be able to because of the needle length as Jan mentions above.
I know a while back Knit Picks addressed the issues of interchangeable 16" circulars. They said that to keep the quality they where going to keep the 16" circulars fixed. Which is fine with me BUT I too wish the 16" where [LEFT]interchangeable that way I didn’t have to buy all of the needles again.[/LEFT]