on the second design, my only concern would be is if you were using a vac pump which I am planning on doing,maybe the rtv and 3 or 4 sheet metal screw's down the center would do the trick,, I like the first design for my application by using an Indy or Mancini center plate with inspection cover, that should seal better with the cast frame and the sheet stock having the break close to the sealing area for stiffness, both would work good if a vac pump were not used.