Captive Tube Question

You want a telescoping leg.  You have two sizes of carbon fiber composite tubing: 1x1 inside at top and .875 x .875 outside at bottom of leg.  You want the outside tube to stop moving when it reaches the end of the inner tube.

Without adding anything to the inside or outside of either leg how would you make it stop?  A captive leg.

There seems to be no obvious way.  I will screw four .1" blocks to the top of the lower block exterior and four .1" blocks to the bottom interior of the top tube.  Or more sensibly, cut two square tube sections that will go in their respective positions.  This simplifies getting everything to stay.  Making the sections involves cutting 1" x .1" wall section and .9" x .1" wall section, then using a .125" mill to remove the interior square.  I can make these from. 125" thick plate