I used this for a Mesa 4x12 for about a decade. It has done it's job very well. It still has many years to go, as everything works just like the day I purchased it. The dimensions are outside of case, but the Mesa 4x12 is an oversized cab, so I'm sure it will work for any 4x12, assuming that's what you're wanting it for. I framed out the inside, and filled the voids with sprey foam, and painted that black, so only darkness can be seen from the stage floor. If you want traditional foam, you'll have to foam it. The interior wood is 1/2" cdx. Notice in the pic, I made a small cut-out, so I could plug in the cab without taking the cab out. The cut-out has a nice seal, and a handle for easy access, and has worked just like it should.
I have had my money's worth, but you have to offer more than the cost of building a solid work bench, as that's what this is about to become, if it doesn't move. You'll need to pick it up, and you'll need a truck.