Thanks all. I want the unfinished, not black, for the D90 by the way. I checked with George in New Orleans and he wants $235 a pair. I think I might go with Dap at $65 a pair. We'll see if I can find a set second hand first...
I honestly didn't even think to use a backing... and it most certainly isn't water tight. But what would be? I suppose some foam that stuck to both sides... Still will always have the chance for water egress I suppose. I hope this doesn't become a problem. What about all the 5-bar people put on the sills, and rear quarter panels, is that all backed with something? Hmmm.. I just don't know.
From my experience, the wings themselves are more than strong enough to support your weight, its just that the soft aluminum will crease and dent. This is what happened to mine.
Now the 5-bar hides those old creases, and doesn't crease and dent itself! So as I expected, I was able to walk on them fine.
Well, it appears that 2mm is fine then if you can walk on them.
For backing I would use a piece of rubber 1 or 2mm thick cut to the form of the protector. I would not dick around with cutting the rubber to fit the outer edge and the inner edge (vent hole), just use the entire area.
I would not use foam as the majority of that is air and can easily allow water in. If you want to do the edge thing you may try some silicone. This would seem to be best as it can form around any irregularities on your tops. The rubber solution will count on the thickness of the rubber to fill in the bumps. If they are substantial then silicone may be the only way to go. Even so, using silicone would leave a cavity between the two beads so I hope the plate doesn't flex! If so, you can put cross-brace beads of silicone to alleviate that.
I would use backing. I see trouble if any water gets under there. Not to mention any dirt that simply blows around. Or even mud thrown on top! Other plate on non-flat surfaces should be siliconed, especially the rear fender corners where you wouldn't be able to get a really tight rubber fit.