Cob pizza oven

So I want to build a cob pizza oven.  It occurs to me that the fiberglass igloo doghouses I see would make a great form to build over   BUT my concern is how to get the oven off the mold after forming.  I'm guessing 3 - 400 pounds, and probably brittle.  

Suggestions/comments/concerns are most welcome.

One method (have no idea who invented it) is to build the hollow part out of sand (possible a little clay to prevent it from falling apart instantly) and then building on top of that. Once dry, you can shovel/scrape it out.