So I don't claim to be the first person who discovered this but I figured I'd post what I've done. It turned out pretty well and I hope to make another one soon. For the inside of the box as you'll see I only did foam on one side. This of course is personal preference.
1-"Art Box" (found at your local craft store. Hobby Lobby for me)
4- 11" x 17" pieces of 3mm closed cell foam (also from your local craft store)
Pinking Shears (optional)
Cut 11"x17" foam pieces in half. Carefully measure out the inside of the box. Take your time and make a good template piece. This part is the biggest pain in A#$ of the whole project but once you have a piece that fits good you wont have a problem! It helps to use some thick cardboard instead of foam for you template.
Take template you made in step one and cut 6 pieces of foam out. (I ended up with 6 which is about 3/4" thick. again up to you.)
Take rubber cement and glue each piece of foam together. Cover with weights until dry.
Once dry take box and spray bottom with spray adhesive. It WILL get everywhere. Just how it goes. Place layered foam in bottom, weight down again and wait till it dries. This doesn't take long.
If you want foam on the lid. Make template of lid piece with a piece of paper or cardboard. Cut the piece out of foam and use the spray adhesive to stick it to the lid.
Step 6 (optional): Use scrap foam (size needs to be width of box) and draw a straight line. Use pinking shears and cut on straight line. Use ruler to mark desired width and cut strip. Hot glue in desired increments in box. NOTE: I have also cut slits in the foam like many fly boxes. This works pretty good as well.
BACK (I'm a sucker for decals)