Since getting sawdust spit in my face was annoying, I've experimented with different approaches to catching the dust. What I've found is the high velocity air flow from a shop vac when concentrated to a small or narrow area right near the blade works best. A large Dust Collector moves air too slowly to work well in my experience. Of course, newer table saws that have blade surrounds inside the cabinet work best, but for older contractor saws, it's a real problem.
It's is very difficult to catch the dust from 12" away inside the cabinet, so contractor saws count mostly on "gravity" and a slanted shelf, rather than dust that's suspended in the air stream to work efficiently. That brings us to the over the blade approach.
What material is cheap, readily available, requires no threading or welding ... 2" PVC.
How do you support the pipe over the blade? My contractor saw have a splitter plate which can be drilled to accept bolts or by slitting the 2" PVC, just slip it over the top of the plate as in these photos:
Slitting the 2" PVC on the table saw is very dangerous, so no pictures provided for liability reasons. It was scary!
Before the Triple 12" saw there was a Twin 12" table saw where I supported the PVC on the end of the table extension using a 2 way fabricated plate clamp and exhaust pipe clamps. It was adjustable in all directions and pivots way when not needed. See it here;
OK, how do you attach the shop vac? Using a 90 degree elbow at the end and a few wraps of electrical tape on the shop vac hose, that was easy.
Different tips were tried at the suction end with mixed results. Having the blade cover act as a blade guard made a lot of sense, so I kept it extended beyond the front of the blade for that reason and for better dust collection. Most of the dust get spun off the rear of the blade, but not all, so the sooner you can catch it, the better.
See post 21 here for an example of the end attachment: