For large, high-Q coils, some crystal set builders use a styrofoam cake dummy like this:
Styrofoam is as close to air as anything solid I know of. It may be tedious to set up, but if you can support whatever you're measuring with fishing line you should be able to eliminate all support interaction. But you still have to worry about other stuff in the room. One time I was trying to accurately measure the Q of a large coil with my HP 4342A Q meter. I was astounded to find that I had to remove myself several feet and crouch down to eliminate interaction with my body. Even then I wasn't sure I got rid of it all. I couldn't read the meter from any farther away.
Brian
