My first is a wooden box that I made when I was a kid for lps and then turned it into a dead box by taking a solidstate cheap guitar amp (a charvel amp with an 8" speaker) I took the whole panel with the speaker on it and put it in the box but first I covered the inside of the box with foam and pillows. I ran my Marshall 9000 pre-amp into the amp section of the charvel amp (8 watts I think) in the days of 4-track recording it did okay, but then I got a sansamp an the box became a place for cables.
My second was in this chest that has been in my family for year and years.

My father rammed through one of the doors with his power wheelchair while trying to learn how to drive it so I figured it had no value anymore. I used fiber board and foam. What I did was put a layer of foam and then fiber board sandwiching the foam...I did this times three on five sides and then made an insert for the front where the doors are. I put a 10" speaker in it and miked it. It kept out the outside noise but didn't allow me to turn up the volume enough and it sounded pretty bad.
I have thought about ballistic gel but I'm not sure about whether gelatin would spoil or rot, I worked with it in the service making a form of napalm (I had an interesting job in the service). It turns out gelatin is used for cheap ballistic gel, but would it rot and start to stink?
Of course air and density stop sound so a box within a box would be best with something thick (I'm think 5/8" dry wall layered at least 4-5 times for each box). Because wieght is not an issue for me I have thought about a box surrounded by cinder blocks on top of some kind of slab surround by wood to look nice.
I also wonder if an old freezer that doesn't work could be modified...so what other ideas do people have?