I sketched up my jig, a circle is routed in the deck just slightly larger than the tube's outer diameter. The deck is clamped at a right angle to the tube and set with large worm gear clamps - not drawn, but you get the idea (hose clamps ).
The router's depth is set to coincide with the placement of the deck (say 1/2" down on a 3/4" deck), and a spiral up-cut bit is used to shave the end of the tube. Sweeping passes with the spinning up-cut bit over the tube. The cut into the tube's end will end up being 1/4" inch taller than where the bottom of the deck sits relative to the tube.