Make me an offer! Don't waste time building a new iso booth, just buy this one. Great isolation, and none of the walls are parallel. The booth is modular, so it can be taken apart and transported (though the pieces are quite heavy. . .which is what you want, for stopping sound).
Booth is framed with OSB and 2x4's, and then all four walls and ceiling are completely covered in the 4" thick, 8lb rock wool. Fits under an 8' ceiling. Footprint is roughly 80"x80".
Includes the diffuser/bass trap/lamp seen in the picture.
Materials alone cost more than $1000.
I will help you disassemble and load into your vehicle (I recommend a box truck).