I've got a pretty good idea of how I'm going to build it, unless there's a unanimous vote here to avoid something. It's got to fit my 2x12 Avatar cab in it standing on its side, and it's going to roughly be LxWxH 32x32x36. 1/2" thick plywood of the cheapest variety I can find (so far that's <$12 for a 4'x8'). Lining the insides will be Rockwool Mineral Wool 60, just a hair over $100 shipped from Amazon for 6-2'x4' sheets. Leaving it at just that, that's maybe $140-145 for materials. I'm about 50/50 on whether that will be enough to soak up all the sound waves. If needed, I could buy about 36 panels of some cheapo 1'x1' studio foam, but that seems to add another $50 so I'd almost like to try it without that first. I think me dimensions are big enough to allow an extra 1" of foam internally. I read another thread where a guy built one but the speaker cab was part of the structure instead of a box being built around an already existing speaker cabinet, and people complained of it sounding boomy. I figure if the cab is separate, I won't run that risk since I know what to expect from this cab and I know it isn't boomy.