Joe recommended the extensible pole by Shurline. When he went to demonstrate it, the nail pin he was using, that had an SB900 on a bracket and holding a Lastolite 12 soft box took to the air and bounced off an attendees head. So when doing my own, I wanted to try an alternative.
The Shurline has an aluminum insert that is threaded for a paint roller handle. This makes the outside diameter of the adapter too large to fit into a small flash bracket. The solution I saw involved removing the insert and replacing it with an Avenger nail pin. I had a hunch that the outside diameter at the inside of the thread was pretty close to the inside diameter of a flash bracket. So I went and bought a Shurline at Home Depot for $30 to see what I could do.
Using a micrometer that measured inside and outside diameters, I found I was right. The outside diameter of the fitting at the inside of the thread was a bit smaller than the inside diameter of the flash bracket I was testing, which happens to be the Joe McNally Tri-Flash bracket from Lastolite.
In the first image you can see the ground down thread section. Its pretty close to flush but not entirely, but is ground from the tip to the mounting ring. This allows the tip to fit deeply into the flash bracket mounting hole.Since I did not want to go the nail pin route, I took the pole into my shop and went at the fitting with a grinding wheel in the bench grinder. Going reasonably slowly I ground down the threads to be mostly flush. I didnt smooth things out completely because I wanted it to be easier for grip screws to grab on.
The Shurline is collapsible and unlike many other options has a simple trigger release and pinhole locking notches so you arent rotating friction locks like on a tripod. While this fixes the expanded lengths to fixed distances, it makes extending fast and easy. Also nice is that holding the handle while pressing the trigger release and pulling apart, the pole expands both ways so its easy to control.
In the next image you can see the pinholes in the pole expanded to the first notch. Youll note that the visible pinhole provides a locking notch for when the pole is completely compressed.
The final image shows the pole expanded a bit with the McNally Tri-Flash bracket mounted on it. Its a simple enough project and much less expensive than the flimsier booms sold for around $200. Total cost was the $30 for the pole and about 10 minutes of my time on the bench grinder. You could use files of course but that would take more time. Some poles have plastic fittings instead of aluminum which will be modifiable more quickly, but would also be more fragile than the aluminum fittings.