I started with a 4/4 thermoplastic violin case I purchased from http://www.musiciansfriend.com
Step 1: Initial Case Interior
The case comes lined and custom fitted for a violin. The material is a type of faux fur that is somewhat elastic. The base is styrofoam.
Step 2: Remove Interior
The lining material is glued to the styrofoam with spray adhesive and and styrofoam is glued in with hot melt glue. It takes a bit to get it started, but after you get under it, it pops out rather easy. A large flat screwdriver or small flat bar would be well suited for this. Don't worry about busting it up or tearing it. It'll all be throw out and replaced.
The only special note is to ensure you get almost all the hot melt glue out. It is an absolute bear to pull out in some places, but it all has to come out. I left a small bit in my first case and it came back to haunt me when I went to install the retention straps and the final lining.
Step 3: Prep for New Base
Once all the old lining is removed ( I know, I know, I didn't get all the glue in this one. It was a major pain later) tape off all the aluminum edges on the interior with painters tape.
The tape will prevent any of the expanding foam from sticking to the aluminum. If it does get on the aluminum it has to be scratched off, marring the finish.
The blue tape will also give a guide as to where to fill the case with expanding foam.
Step 4: Add Expanding Foam
Start spraying the expanding foam into the case trying to stay just barely under the tape line. Once the foam expands and cures it will rise above the tape in places, but it can be trimmed down. One can of foam got me about 75% the way through. I had to make a mad dash to the hardware store to get another and the first section had already started to cure. The remaining foam didn't quite adhere as I had hoped it would, so, lesson learned. Have 2 cans ready.