I built one several years ago. I used ripped 2x4s for the frame, and MDF for the walls. I think I just put regular old insulation between the walls. It's bulky, and weighs a metric shit-ton, but it actually does a really good job of knocking the volume WAY down. I mean, like a 50 watt tube head can be cranked and you have a conversation at normal speaking levels standing right next to the cabinet.
The egg crate material is just a foam mattress pad. When it was bare walls inside, there was some horrible resonance getting picked up by the mic. I used a short gooseneck mic snand screwed to a podium mount. The speaker is an old Peavey Scorpion. I don't know that I have any clips of it.