I've been looking into building a vocal booth as well. I came accross these plans, which could be an interesting option:
www.projectstudiosolutions.com
I haven't seen the actual plans yet, but they seem pro in terms of non-parallel walls, ventilation, materials and able to be dis-assembled and re-assembled again and again etc.
Let me know what you think. In anycase, keep us posted on your progress - I'd love to learn from your mistakes
Rez