Glad it worked for you, Sam...and appreciate the feedback. I'm trying to get the word out as much as I can...I'm finishing up an article for Kitplanes on the subject. As part of it, I've been on an adaptor-building binge, trying out a number of different approaches.
For those who would like to try this out with minimum effort/minimum cost, here's what you can do: Start with a 1/8" to 1/4" earphone plug adaptor. These allow plugging standard consumer-grade headphones (with a 1/8"/3.5mm plug) into the standard 1/4" headphone socket. They usually come with upscale headphones, or you should be able to find them at places like Best Buy. They sell online for as little as a buck.
Using a drill, put a slight dimple in the insulation between the tip and the first ring. Then use solder to bridge the tip and first ring. Sand down the "bump" from the solder so the plug will still enter a jack.
This converts the adaptor to mono...puts the same audio signal into both the left and right channels.
Now: Take an ordinary set of ear buds and plug them into your handheld radio, using the adaptor. You should be notice one heck of a lot more volume.
If you want to try it out in flight, just put your regular headset on over the ear buds, and insert the microphone plug into the headset adaptor. Use tie-wraps or a bit of tape to keep the unused headphone plug out of the way.
You can try this out WITHOUT modifying the adaptor, but you'll have only one ear bud working.
Now, if one has an intercom in the airplane, it's likely you've been able to use the intercom's amplifier to crank up the volume to overcome the mismatch. But I heard from a gent in California who had problems with the levels...with a second headset plugged in, the engine noise would be picked up by the mikes and played over the headsets.
He put the impedance matching transformer between the handheld and the intercom, and the problem went away...he didn't have to run the intercom volume so high.
Ron Wanttaja