I have 0 experience with mobile WFO's. Your photos are pretty small, but if the chimney is within the cooking chamber, which it looks like it might be, then I see that as a serious, possible fatal, problem. The whole issue of air flow needs to be just right so that you can support a nice,hot, non-smoldering fire, but that as much of the energy is trapped within the cooking chamber heating the decks, wall, and dome. Heat escaping through the flue or opening needs to be minimized. This balance is one of the most important aspects of a good oven design. If you try to decrease draw out the chimney, you could choke off the fire. Give it a try.