The way I built a vocal booth was out of FREE!! pallets packed with insulation. Those heavy duty ones that are painted blue. Most store have piles of them in back and will give you one or two if tou ask nicely
For a 6'X8' isolation booth:
Put 4 pallets down and you have a 8'X 6' floated floor...I put 1/4" plywood on top of that.
Take another 2 pallets and and turn them up right...the 3' open end running vertical and the 4' closed end going Horizontal. They will span 6 feet wide and 4 ft tall. Center them on the 8 ft floor and nail these 2 pallets together and to the floor pallets with nails and you have the beginings of your front wall. This will give you a foundation sturdy enough to support the window.
For the window you go out and find these junk dealers who sell salvaged windows and doors. Most if them have a lot of sliding glass patio doors that the bottom have collapsed springs or doors that don't have a track...in other words they're basically worthless. They go for 5 bucks a piece. You should be able to score two nice thick glass metal framed patio doors for 10 bucks. They usually run aprox 7 feet long so you Will need two 12" 2X6 pine boards to frame the window. Carefully measure and frame the window ....center the frame on the upright pallets and nail it to them. The frame will be a few inches wider than the upright pallets you are nailing it to.
Decide how tall you want the isolation booth to be and get two 2X6 boards that length or cut them to the desired length that gives you the height you want your boothe to be and nail them to each side of your window frame and to the pallet floor .
I chose 8 ft as the height because that way I could do my entire back wall with 4 pallets nailed together.
Anyway then you frame your corners and your side door and your side wall...4 pallets for your back wall. 1" pine lumber slats for the top frame (to support a Sheetrock top)
Use that small half inch shoe molding around the edge of the window frame... Push one window in and bump up to it. Nail shoe molding on the other side of that glass (the middle piece that separates the two glass doors) put several silicon packets in and bump the second sliding glass door up against that molding and then nail molding around the frame again to hold that door in place.
pack all the pallet cavities with insulation and then Sheetrock the outside. Use whatever you like to line the inside.
I used thick linoleum tiles and carpet over that.
It was very sound proof when finished. I could crank
a tube amp up in there and you could barely hear it outside my studio