On Sun, 18 Dec 2011 10:19:43 -0600, HeyBub wrote:
Assuming you've found the 3.5" OD pipe, all you need is a way to attach
your 2" mast. Threading with a reducer is one way, so the next step is
find some way to put threads on your 3.5" OD pipe.

Sorry about that last post - I hit the send button too soon!
I'll clarify.

Local plumbers told me to look for 'ballards' or 'bollards', which are
apparently inexpensive pipe posts ($50) that are about the right diameter
which are designed to be used as "dissuasive perimeter control" barrier
posts around areas so that cars don't drive through them.

This site calls them "Pipe Ballards" but they only have 3" diameters:
http://www.bryfab.com/bb.html

This site calls them "Steel Bollard Posts" & but they only have 4", 5", &
6" diameters:
http://www.absorbentsonline.com/indu...steel-bollard-
posts.htm

This is a bollard FAQ:
http://www.bigbollards.com/faq.htm

I'm still googling for a 3.5" OD bollard/ballard post ...