Building a guitar iso booth. How is this for a design?

First off let me say this... this forum is amazing. I am not sure I have ever seen a web community come together and help each other out so much in all of my life. I wish all forums were this pleasant to read. Ok... here is my deal and my dilemma. I am an engineer in Nashville. Most of my work is mixing but from time to time I need to record guitars and vocals. I just got kicked out of my studio where I could be as loud as I wanted at all hours of the day. I am moving into a house in a residential neighborhood and now I need to worry about ticking off my neighbors. I have a room that will be great for vocals once I put up some OC703 but my main dilemma is that I want to be able to record screaming loud guitars and not bother anyone. The majority of the players that I work with are PRO session players who have their tone. If I asked them to plug into a POD I would get laughed at and not hired again. Plus I just think that they don't sound very good. I have done some extensive searches here and elsewhere and have yet to see someone build an isolation cabinet big enough to put a 4X12 cab into. Size is not an issue for me... I don't mind if it takes up the whole garage. However I do want to be able to take it with me when I move to a more permanent location. What I am looking to build is a 4x6x4 box that I could fit any amp into I wanted. I want it to be big enough to give the speakers some room to breath. Inside this box I am thinking of lining it with about 4 inches of 703. The bottom would be filled with sand probably and I am not sure what to do with the top. If I built a box that one layer of 3/4 inch masonite and 3/4 inch plywood, then a four inch air gap, then another layer of the plywood and masonite would this give me much insolation? Your thoughts and ideas are worth their weight in gold to me. I have built studios before and worked in some of the finest rooms in the country but I have never seen an iso box like I need so I really have nothing to gauge it off of. Please keep in mind that a demeter cab or anything like it are not an option as is running through an amp simulator. These players have cabinets carted in for them... sometimes... many... they are very serious about their sound. I have attached some crude drawings of what I am looking to build. Thank you in advance for your help!! Ben Strano

I think you'd want your amp iso box to have 'golden room' ratios. I don't know the numbers offhand but they can be googled pretty easily and will help balance the modal response of your box. The real challeng I beleive is if you have to contend with combo amps...now you need find a way to ventilate the box to prevent the amp from cooking itself. I'd say forget plywood...MDF and drywall would be the way to go. Mass is your friend! And pick up some green glue to damp out the wall resonances. Actually, maybe MDF exclusively is the way to go. That way you can unscrew the whole thing when you need to move. For the bottom of the box, how about mounting it on hat channel held by RSIC clips. You could also use the clips for the top and interiour walls of the box too. The door will be a challenge...and probably the weakest spot of the whole thing. Cheers Kris