I forgot to dig out one of my towers this morning for pictures but imagine this:
with a lopro on the open end. The only difference is that Mark sold me just the head (not welded to the tube). So I could bolt it on to scrap NRS pipe. I did this, so that I could adjust tower height on an ever evolving frame project. In the end, I found what worked was 8", so I bought the new shiny NRS 8" tower. The drawback to the bolt together version was that it was sloppy. The two bolt together joints each had a bit of slop...nothing horrible but if I stayed with that system, I would have welded it together once I settled on a height I liked.
My other version was HDPE rod inserted into the end of the pipe with two bolts holding it in. Exactly how Zach describes but with plastic instead of aluminum. It worked fine too, but I didn't like not seeing my keeper pin in place and as Zach said, to change locks it required some disasembly. That was the cheapest option. I will post pics of it as well latter. I went from the AAA version to this so that I could narrow up the lock width a little and get a better angle for my oar lock. To make the lopro not dig into the top of my tubes I had to tilt the towers out about 20 degrees, add 20 degrees fro the AAA hed and there was too much angle, or flip it around and there was 0 angle... I hope that makes sense.
In the end I only see a couple reasons to do DIY towers, one is to go with a non-standard size, the other is to accomodate some funky angle issue or the like. When it's all said and done you will pay nearly the same DIYing as buying production towers...
There were some other threads on the subject and someone made a loop system that used two lopros. That would be another potential advantage, redundant/stronger connection.