As to the filler I'd suggest using a lot of RTV on the top side of the gasket (some underneath too, but a whole lot on top) then try to clean it up once the wing is in place. I used a little, but it left wide gaps around the base, so I used a tube of sealant left over from doing headlight motors to fill in that space. But that's a very messy job once the wing is in place.
For holding the wing on I fashioned a piece of aluminum and used a sheet metal screw from the underside to grab it. So far so good. It seems like the RTV is doing as much to hold the wing in place as the glued-in nuts. The three remaining nuts sure don't look like they could resist much pulling.
Here's the strap:
I also sealed the underside of the trunk lid. This is the one with the sheet metal screw and aluminum strap: