The foam is simply styrofoam sheets. I have made pans out of heavy gauge galvanized steel. FYI, you don't really need 20gauge metal for this. Don't ask how I know this...
Now would be the time to add a grey tank, which sits next to the black tank in that rear area. Ours is ten gallons, but you can easily reverse the sizes of the tanks, and have a ten gallon black, and a 25-35 gallon grey tank in that spot. You can go with a pair of 20's as well.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.