DIY isolation box for guitar/bass amps/cabs?

[Superhuman]

Hi guys,
I'm in the process of building an attic studio and I have access to a carpenter and builders materials at cost so I was thinking of making the most of the opportunity and creating an iolation box for recording guitar/bass. I saw something like this before in a book and it was called the 'monster in a box', you basically make a heavy duty soundproof box, set up your amp/cab inside and get the mic set up and then seal the box to keep less sound from getting out.
This could be the solution for me given that my studio is in a semi-detached residential property. Anybody have experience building one of these or even better have plans for getting the most bang for the buck? I know it won't be anywhere near completely soundproof but its all about reducing the noise transfer while giving me a more comfortable environment to work in (I already have tinnitus from playing too loud over the years).
I figure that if I can get something like this working then I can crank a tube amp or bass amp in the isolation box and the treatment and room itself will stop the sound from getting out into the rest of the house (recording late at night while the wife and kids are sleeping).
Any advice would be cool even if it is just to drop the idea!