I have seen UDS's that work fine with several small holes in a flat lid. Some that even just had pieces of steel or magnets to cover the holes. I agree that a larger chimney would work better. But, you have what you have. Go for it and see.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."