The layering technique that's described definitely works, but I'd replace the acoustic foam with some Rockwool or Owens Corning 703. This is the same stuff that's used in recording studios and if you use Rockwool, it's a lot cheaper and much more effective than acoustic foam. You can find out a lot more about what it is and where to find it from this post on acoustic treatment, or from The Studio Builder's Handbook.
1. Make sure that the seal on the box is absolutely tight for max isolation. Air is like water and if any can leak out, there goes your isolation.
2. If you place a combo amp inside, it's going to heat up because there's no place for the hot air to escape, which can end up causing some major damage. You're better off using just a speaker cabinet inside.
2A. Players tend to crank their amp up all the way when you get as much isolation as an iso box can offer, but be careful. It's really easy to overheat your amp head even if it's not in the box. Once again, this can cause considerable damage.
Help support this blog. Any purchases made through our Amazon links help support this website with no cost to you.
You should follow me on Twitter for daily news and updates on production and the music business.
Don't forget to check out my Music 3.0 blog for tips and tricks on navigating social media and the new music business.