1up -
Why did you have to sand down the riser?
I didn't actually sand down the 1/2" sprinkler riser - I just sanded the 1/2" PVC coupler between the riser and the galvanized reducer/coupler.
I used a 1/2" male-male (ABS?) riser that fit inside the handles with no problem.
me too
I painted the whole thing black - otherwise you'd be able to pick out the dark grey riser and the white PVC coupler (I should have stated that above).
What diameter was the nipple? I find that 3/8" OD pipe fits into a Happ super joystick base perfectly.
The nipple diameter was 3/8" - but being galvanized it had some anti-rust coating that made it a *tad* larger, which wouldn't allow it to fit into the Super base without *a lot* of coercion. I ended up making about 6 handles/bases because I broke too many in the process with hammer/vice. Yeah - I get frustrated some times...
How is the nipple attached to the reducer coupling?
With a regular 1/2" PVC female/female threaded coupler
How do you keep the stick from spinning around in the base?
Friction

Actually, I was going to rig up a stop, but the wires were pretty tight and I taped everything well, so the handles resist turning fairly well. Of course, if someone really wanted to, they could spin it 4 or 5 times and pull something out...
Any problems with the coupling coming unscrewed?
No, that thing went on very tight (like plumbing should). It was actually very tough getting everything apart. You know how it is when you are engineering something for the first time - trial and error up the wazoo. I have full confidence in the handle strength...the felt quite stout after assembly was complete.