As Bill Cosby would say; "Wait til them kids in those Chevy's see these PIPES! Dual chrome exhaust tips - LOVE IT!
So - inside the firebox build a small box that just covers the two side-by-side outlets for your stacks and put your baffle in there - just adjust it when you have the door open. After running Edgar for about 4 cooks now, I've learned you don't "fiddle" with the stack much anyway once you're onto how to run it.... or you might put on an exterior control somehow ...
Another way is to build half of a vent iris into the stack and the other half into the chrome stack and just rotate the chrome stack to get the opening you want - completely invisible ... unless you're using angle cut stacks ... then it might look a little weird - but straight, rolled or flaired end would be fine.
Chrome stacks should be removed after the cook, wrapped in satin cloth and kept in your safety deposit box anyway - so just put a couple tin cans over the remaining stack pipe to keep the snow and the birds out. This might not be helpful but it sure was fun thinking it up!