Well, I finally did it. The engineer in me just rebels at the "state of the art" in applying goop all over the mounting flange to cover the mounting screws, and then more goop to cover the flange to deck seal on top of the butyl. So. After making up a stainless steel mounting ring, and getting my sealed screws, went from this:
Note: that dirt was carried there by water...
The stainless trim protrudes over the stock mounting flange by 3/8". So the additional deck seal covering the stock flange to deck joint is now 3/8" wide by about 1/4" high, UNDER the stainless trim. The only thing I may redo is that the inner joint has a bit too much sealant to proud. But it was getting late, and I was on my last tube of Dicor, that had already been cut to fit the deck to stainless gap.
I had already installed the MaxxFan flange, using Dicor non sag; so I stayed with the same. Based upon some other comments on the site, think I will do the front fan using Sikaflex. Will be an interesting test to compare them side by side for durability.
Next is to re-engineer the skylight mounting goop...