That is styrofoam! Really hard stryofoam that can support over 2000lbs without moving 1/8"! Simply cover it w/ firebrick and mortar into place with Hi-Temp mortar.. allow to dry.. and you have an oven. Of course you will want to add ceramic fiber blanket and a fancy exterior.. but the foam provides you with the perfect inner shape & dimensions. To remove, simply cut it out with a hand-saw. And Thank You for the compliment! I tell my wife that I am a genius all the time.. maybe now she will believe me!

Thanks a million for being so generous and sharing your knowledge. God Bless!