My dad bought me an Audix Fireball microphone for playing amplified harmonica. These are nice mics, with a really hot element. My only problem with it is that it's hard to get a decent seal with my hands around it. Inspired by people I've seen using cups and/or drinking glasses for special effects and for practicing holding something about the shape of a traditional harp mic, I decided I'd build a "mic in a cup" to try to get a better seal. It worked great, and was very simple to do:
Now you've got a mic you can actually get a seal on. I'm definitely able to get a better overdrive of the mic's element now. Here are a couple short quickly-whipped-together sound clips to illustrate the difference between accoustic playing, the mic-in-a-cup fireball, and my homemade pill-bottle mic -- the latter two played through my Fender Blues Junior amp. All of these were recorded through my laptop's built-in mic with Audacity software on the same settings, so the level you hear reflects the difference in the mics. I was just sorta noodling on the harp, but I tried to keep the noodling to be at least a bit similar for recording each of the sources.
(those were all recorded with a Bb Hohner "Special 20" harp)
To make a pill bottle mic, get yourself a cheap dynamic computer mic, a standard quarter-inch phone plug, and an appropriately-sized pill bottle. Drill out the lid of the bottle in a fancy pattern, disassemble the original microphone to get the element out, and glue and/or tape it into the lid of the pill bottle. Solder the wires to the phone plug, and you're done.