What kind of red clay brick are you using? Just trying to save you from paying twice, but cast refactory concrete is going to be much more durable than red clay brick. Even if you had fire brick, I think it's a very dubious task to do a mobile brick oven on the cheap. Depending on the red brick you have, durability issues aside, you could end up burning a ton of wood in that thing just to get it up to temp. I can appreciate wanting to do it on the cheap, but you'll probably end up using three bags of refactory mortar just to use the brick the brick you have.