Water Heater Oven

[MarkY]

I started making a mortar-less pizza oven, which was very successful, but have been thinking about an easy way to dome, or barrel, the roof. I had an old water heated so I extracted the core, cut it into a half-round (with an end) and laid the bricks over it. The first burn wasn't great (not enough air) so I raised it on one course of bricks (and added a lower entrance) and the next burn went much better. I have one question for you: Can you think of why this is a bad idea? I know the metal is coated (but I believe it's a ceramic / glass coating that shouldn't be toxic when heated), and I suppose I'll get some rust once it started to corrode (I see a lot of old heaters beside dumpsters, so I can replace it when needed). Obviously the oven won't stay as hot as a normal mortared oven, but my previous mortar-less oven had no trouble getting hot enough for pizzas.
I look forward to hearing your thoughts.