A while back I built a 'vocal booth' after reading some posts/articles here on the net. I made mine like a 7 foot cube out of Sch.40 PVC. I triple covered it, top as well, with the heaviest moving blankets I could find. It is by no means sound-proof but it does dampen some of the outside noise and it has added a 'new' sound to my vocals that I really like... I am a little leary of the plexi-glass walls though... I wanted something that would absorb, not reflect... Just my .02...
Thanks,
Zetajazz44