noob to the forum be gentle with iso booth questions

:D hey hey I stumbled apon this site a few weeks ago and now I need the help of you guys for a few things. I need to find plans(free preferably) for building vocal and 4x12 guitar isolation booths. my big concern is that if I build the rooms in 5x5 configurations with 6 and1/2 foot tall ceilings(I am putting these in the basement of my new house) I will be auralexing the heck out of them since I recording metal almost exclusivly there is no need for these two rooms to have any "character" or ambience. I just want to avoid a "phasey" or "boxy" sound from the rooms. any ideas or help would be wonderfull!!! Love Curran