I used a 10" diameter sewer pipe for the tube. It's very heavy but stout. I never have to worry about it being crushed or dented and if it ever gets a scratch, it can be easily sanded out. All in all it works very well.
The tube is lined with black felt that I purchased from ProtoStar. I followed Jim Fly's instructions on how to install felt in a telescope tube and it came out pretty well. I had to make use of my brother in law Danny Hughes' long arms to get the middle of the tube since it was just out of my reach.
I
fashioned two 4" diameter pipe plug caps into trunnions by first cutting
them to roughly the shape of the 10" tube and then by rubbing them against
sandpaper taped to the outside of the main tube. This yields a perfect mating
with a professional appearance. The insides of the caps are filled with epoxy
and plywood to make them strong and stiff. I contact glued strips of formica
to the outsides of the trunnions where they rub against the teflon blocks on
the mount.
Click Here to see details on how to cut trunnions for a perfect fit.