With all the openings and changes in elevation I figured I would concentrate on keeping the airflow moving, hence the reuse of the sealed corner. That and the U-Turn area are really the only sealed areas on the OEM, so I just matched it using your description of the airflow.
You did the same by making it all flush. I was actually going to do the same as you did by putting a silicone gasket between the layers (and still might) but once I saw I could get the 2 areas sealed off, I called it good! Not sure how much extraction that the tunnel dip will provide. I'm limited there as mine is a full bore hole a couple mm down from my grinding. So I opted for increasing the velocity to get the most from it. A big 45 degree change would have been nice to use there.
My adapter actually only had a step on the normal side, so everything on the right is suspended between the tunnel where I ground it flat and the far right where I notched it like you. The "blocker" provides mid support. It's all held tight by the bolts on that side. That's why there is 2 of them over there.
We'll have to compare notes after I get a few miles on this set-up. I'm sure they'll be close.
It'll sure be nice to stop seeing as much oil in the can!!
Ken