Rear Chimney Earth Oven?

Hello all,

I built my first Earth Oven two years ago, and last year built another four. I am looking at building a community earth  oven at a park picnic shelter, which has an existing shelter with charcoal barbecues with chimneys. Ideally I would tie into an existing chimney, but that would mean building a chimney in the back.

Anyone have experience with building  a rear chimney earth oven? How was the burn efficiency? Was it much of a pain to build a damper in the back?

Thanks for any help you can give!

Jake