Little time after I got off work this morning
Got the dunnage out from under the topper, got it set in position, and then rigged up some scrap bracing to stabilize it while I suspended it with the screw jacks. Got the surfaces cleaned with acetone, and put down a strip of butyl tape where the flange sits. I had pre-drilled mounting holes 6"OC around the mounting flange perimeter, and installed 14 gage tek screws to mount
Tony, you better be using SS screws or weld it. Those coated steel screws are going to be a problem with corrosion.
Even though I haven't seen much problem with galvanic corrosion, I did a little google-****, and found that in a steel-to-aluminum situation, the doner material in the galvanic reaction is the aluminum, the zone of reactivity is small, and the reaction is much less active without water. Rusting of the fasteners, clearly, is also much less of an issue in the absence of water. If I have water present at the flange mount, I have bigger problems than galvanic corrosion, or rust. With the butyl tape under the flange, and a bead of high quality sealant at the outside, I think I'll be fine.
While it appears that I have some water damage at the window openings already, (which happened when it was at the painters house without the windows in it, and no cover - Grrr) it is just surface discoloration, with no structural impact. More reason to get it covered so I dont have to look at it
Windows and sealant next