VoiceOver Booth - plans?

Are there any plans available for a freestanding voiceover booth, that has been tried and tested and that is excellent at stopping external noise getting into the mic? I would rather not pay out £3-4k for a ready built one, and as i am reasonably handy, wouldn't mind the challenge. The problem is that my edit suite (its for a tv commercials dept) has rather a lot of noise pollution coming from both airborne and via AC units, as well as structurally transmitted noise mainly coming from freight elevators that are in the building. The real low stuff i can filter on the mic/preamp, but i really need to lower the overall noise floor for the voice. I don't have the time to research and learn all the ins and outs of acoustic design, hence my request for help in finding a generic voice booth plan. thanks adam