You may have a hard time finding a 72"small diameter tube. But a inside ferrule joint can work well. Wrapped carbon fiber is very stiff if desired. I've used 8P tube described at
http://www.skyburner.com/skyshark/p-series.html
Also take a look at the RC mast tube available at
http://www.midwestmodelyachting.com/CFMastTubes.asp