I built one. It's enormous, and takes up a considerable chunk of my garage. The outside dimentions are approx 4' long x 4' wide x 6' high, plus about 10" of height for the base and casters. The dimentions were tall enough inside to stack a pair of 4x12 cabs (no head), and outside widths at 4' because it meant I could take whole sheets of plywood and cut them for length to make the sides. I got to cut a single sheet of plywood in half for the floor and roof. For the base I made a frame with beams like a floor built above a basement, and built my plywood box on top of that. The whole front wall opens as a door.

I started the interior by laying several sheets of drywall on the floor. I think I put some lame sheets of foam under each layer to prevent the layers from rattling (just in case). Then I put a layer of (relatively) hard foam on top of that to float my inner structure. The inner structure was a plywood floor supporting soundboard walls that slanted in slightly that I then built a skeleton for out of 2x lumber. I assembled this outside the box, and set it in once it was complete (somehow). It only has three sides (leaving one side to be closed up by the door) and its cieling which also slants.

At this point I ran my wiring. 2 balanced mic lines and 2 speaker cable lines in one side of the box, and 120volt AC on the other. The holes I drilled in the outside and inside structures are staggered several inches so the lines go through one layer, run inside the wall a bit and then come out through the next layer. I let the lines end in free-floating jack boxes that just sit on the floor inside. Outside, I drilled a hole through the wall from my garage to the studio room. In hindsight, I should have run 4 mic lines instead of 2, in case I want to use one to run my guitar signal to a combo amp and have more than one mic on the cab. The mic lines will carry a guitar signal out as well as a mic signal back.

Then I filled the space between the inner and outer structures with aerosol foam insulation. I got the one that is still soft when it hardens. I had to drill tiny holes all over the inner structure to get it in, and it took waaaay more cans than I thought it would, but it did the job really well. The soft foam keeps the walls from resonating. I put a loose piece of thick carpet on the floor too.

On the inner surface of the door, I suspended a sheet of drywall from thick picture hanging cable and glued 2" thick foam strips in between them. The inner structure of the box sits about 2" back from the opening of the outer plywood box, and when you close the door the suspended drywall goes inside the plywood outer box and presses up against the inner box making a seal. The foam strips in the door act like a spring and keep the seal tight.

That's about it. I'm no acoustics expert, but I tried to make a sealed box inside another sealed box, with dampening material in between to cut resonances. The heavy, soft, adhesive foam spray made the otherwise horribly resonant plywood quite useful. The amount I angled the surfaces in the inner box is probably just enough to make my construction job really hard while gaining little or no acoustic advantage. The relatively soft walls of the inner box probably help to cut down on reflections, though. So far I have no complaints about the sound quality I get. I put on the wireless and piped it into my Soldano 100watt tube head turned up to about 3 on the hi gain channel, feeding a 4x12 in the box and then walked around outside at midnight to see how loud it was. I could hear it, but the sound of the strings on my strat was easily twice as loud. No neighbor will ever complain about that, and none ever has, even with me recording death metal at 3 in the morning.

Hope this gives you some useful ideas!

Thanks for that Bicasaur. You took some time putting that together. I was not thinking so much about sound proof, just isolation but you bring up some good points. (Late night tracking), Which are also important to think about.

IDK if you have neighbors next door/upstairs or family in the house they will still here it, some what.

Now that I think about it a few of my friends use amp sims for just that purpose!