How to make 6 to 14 extension forearm mechanism ??

Hi!

I am making a cylindrical coordinate arm. I am using an rc servo and plastic turntable as a revolving base. I am using an acme/lead screw and stepper motor for the lift mechanism. For the forearm I need a mechanism that extends from 6 to 14 inches. I thought that I could stack three 6-inch pieces of channel/framing stock such that they hang and slide on each other. One profile that came to mind was a T on top of a C whose opening is downwards, like: _ | |¯| - -

Obviously the above drawing is not to scale in any way, but it shows that 3 pieces with that profile can hang one from the other, by having the T at the top of one piece inserted and slide on the downward C at the bottom of another piece.

Anybody knows of a channel/framing plastic or aluminum extrusion with a similar/equivalent profile??

Thanks in advance.