One of the possible drawbacks to the PVC camera mount is that the entire thing is only held together by friction. Theoretically, the camera mount, and the camera, could slip out of its socket and then bye bye $3,000 digital SLR.
The first solution is to never trust an expensive camera to my setup. I mean, it is homemade out of a dollar's worth of hardware. If you can plonk down a couple thousand for a camera, you should be able to afford a $50 tripod.
The other solution is a locking mechanism that I haven't tested extensively, but seems to work pretty well.
The mechanics are simple. Drill two holes in the camera mount on each side. Do the same for the PVC fitting. Align the holes in the two pieces, then bend a coat hanger or bike spoke so that it fits snugly into the holes. Slide the clip into the holes, and there you go -- a secure connection that won't slip out no matter how hard you shake it.
One thing that I've found useful is to make the wire clip off-centered a little bit. It helps make slipping it into the holes a little easier.
Having shown this design, I want to point out that I probably won't be using it normally. It adds one more element of hassle, and the basic mount is secure enough that I've never worried about my camera falling off.
However, there are a few designs I've got running around in my head where a little extra security might be nice.
Price: Free, if you've got a metal coat hangar lying around.
Build time: 10 minutes, with a drill, wire cutters, and some pliers.
Availability of materials: Readily available.
Durability: Medium to High. The metal clip may fatigue eventually, but it should be fine for a while.
Functionality: Quick and easy, works great.
Portability: Only takes slightly more room than the camera mount.