5 sided box. 2 x 2 frames...build 3 frames with hinges or bolts and nuts. Build one for cieling and if you need it, a 6th one on neoprene pads for floor. Or just use padding and carpet on existing floor. Use solid core door for 4th wall. Use weather stripping around door/jamb/threshold. Line assembly joints with Sill foam. Shieth exterior all frames with 3/4 MDF. Glue and screw. Line with 703/fabric. Vent at your discresion and ability. Real easy. Won't be an STC champion. Cheap and easy are its attributes. There are a million variations. They all have limitations. This one is HEAVY too. But mass is what it takes, and MDF is stronger and heavier than one layer of drywall. Have the MDF cut at a shop. This is EXACTLY like a big packing crate. Beats blankets by miles.
Anything more you might as well build a 2 leaf/ 2 wall assembly room in a room and float it cause thats what it takes for maximum iso. And that is REALLY HEAVY But if its main criteria are being able to disassemble and move it later, cheap, somewhat ok at iso, and somewhat ok for absorption...well, what are you waiting for..

Ha! Heres a step up, but its heavy, non movable, and not real cheap.
Nextstep up is EXPENSIVE, and permenant. But if you think about it, what Steve recommends is pretty much a room with in a room but it will work great, but not portable by any means. How big an iso booth do you need? 4'x4' is an absolute minimum. Even then, can you say claustraphobia? Even 4' x 4' will take 5 sheets. Try and lift 5 sheets of MDF. Ha! I'll post plans tommorrow night. Here is a more robust version. 6'x6', with NO treatment shown and just a plan view. No sections or elevations. Perfect standing wave box, ha! Steve, just tryin to help cause you are so busy with work. If this is out of line, just say so. Besides its a good drawing exercise anyway. Well, my .02
fitZ

alright, breaks over , back on your heads......