Came across this tonight, seems like it may be worth a try-Ive heard that singing in a closet sounds pretty good but never tried it. Ill bet this could mic guitar amps pretty well too.
Im picking up a mic and preamp tommorow for vocals, I may give this a shot.
http://www.instructables.com/id/Portable-Mini-Vocal-Booth/
Looks like it could be worth trying