Hi Bill!
I used 1.5 inch pvc. the pieces are glued into 4 sections. For instance the stern piece is one section, there are two longitudinal runners which attach to the front section. The four sections "slip fit" into the mating section. As you know, you can purchase pvc fittings either threaded or slip style. I used all slip fittings because eventually sand gets into the threaded fittings and you're screwed! If you want I'll take a few photos of the sections.
The sections fit tightly the below the curvature of the tube, and the tube pushes them onto the floor. The front section, fits around the tube, above and below the tube, so the tube holds that in as well. For instance you could pick up the boat just by lifting the pvc framework. In front of the "Front Section", the fuel tank, ice chest and anchor bucket (5 gal plastic pail)wedge the framework to the stern.
A length of mule tape from the stern section to the front section helps keeps things nice and tight.
BTW, make sure your rods fit the 1.5 " pvc!
The 14 foot boat rod holders are a bit more complicated if you want photos of those.