Intake sealing and blocking the heat crossover

I guess i'm having a hard time visualizing things. If I used a .020" piece of SS shim stock, or got the Mopar gasket set with the supplied shims that you trim to fit, how can I be sure that's reliable? It would have to be slightly larger than the heat crossover opening on the head to cover it, and then there would need to be enough gasket material to seal around it. I just can't visualize that sealing well in my head, plus the additional .020" thickness possibly causing port seal issues on the whole manifold.

Is it not a big deal if the shim stock isn't surrounded by gasket? Will it flex enough to seal and not allow an exhaust leak?