How does $25 for a boom pole sound? I just made a boom for the shotgun, and it's working nicely. Admittedly I stole the main idea from the internet, but modified the design to suit my needs.
I bought a light-bulb changing pole (complete with "grips" for different bulbs -- it was a kit) for $19.95 from Home Depot. Also needed to purchase a 3" long x 3/8" bolt, a wingnut, washer, and a rubber grommet. Okay, the total was somewhere around $27 with tax.
The pole came with two interchangeable quick-mounts for the different bulb changing pads. After cutting the head off the 3/8" bolt, I screwed it into one of the quick-mounts. Fit perfectly, but I also used a hot glue gun to completely fill in the tapered hole around the bolt. Next I threaded the wing nut onto the bolt, followed by the washer and rubber grommet.
The Pearstone shock mount for the shotgun simply threaded onto the 3/8" bolt. I don't try to tighten the mount onto the bolt; instead I use the wingnut to snug up the works. That puts the stress between the wingnut and the bolt, instead of risking breaking or stripping threads from the plastic mount.
Finally, I used a few velcro cable ties I had in the odds-and-ends box to secure the cable to the boom. I didn't try to run the cable inside the boom like the $250 units do.
I was going to paint the pole black, but realized that appearance doesn't affect sound quality, and the paint would put me over the weekly allowance my wife gives me.
If you want to use a standard mic clip (holder), just use the bushing that comes with most mic clips (shown in one of the photos). The bushing will adapt the 3/8" bolt to the size needed by the mic clip. I opted for the shock mount because I didn't want any noise from handling the pole to transfer to the mic. However, I've not tested the shotgun using the boom with its furnished mic clip.
In use, the pole will telescope out to around 15 feet and remain pretty solid. I definitely used a high-quality mic cable because I didn't want any noise introduced from that.
Last ideas: Standing around for a couple of hours and holding an extended mic boom is a total drag. I'm going to make or buy one of those flag pole holding belts like the flag bearers use in parades. If you're using a regular mic stand to hold the boom, be sure to counterweight the boom, and use sandbags or something to weight down the stand so the mic doesn't crash into the crowd.