Summary

The original clamp on the umbrella cracked, so...

The second one I printed (both umbrellas' clamps broke the same way... can you say design flaw?) without support, and it was fine.

Instructions

Print the main part, upside down, with support (see photo); I also printed it with a brim. (Update 11/29/2014 - I printed a second one without support, and it printed fine.)

Print the reinforcements on their flat side (of course).

I found that due to the direction of the layering, and the stress direction, it cracked a little at the lever hole, so I made some "reinforcement" parts (that are layered the other direction), and glued them on with ABS glue.

You may need to sand the inside barrel a bit with a drum sander (drill) to get a smooth, snug fit on the umbrella pole.

I'm still using the original lever, and pressure plate.

I had to drill-out/cut the original rivet attaching the lever; I replaced it with a screw (with nice knob nuts -- from a lamp, I believe). A normal screw with a nylon lock nut should work fine.

Tommy Bahama umbrella clamp replacement