Hi Francois:

The .045" aluminum spacer that I was talking about is hollow at all the slot openings. Also the fiberglass deck of the hull will have corresponding cutouts in it.
This will bring the photo-etched decks to their proper height. Also all the slots in the photo-etch decks will be able to pass air/water into the U-boats free-flood areas, just like the real thing.
I made the aluminum spacer as follows: I photo-copied the LaRue decks. I then glued the photo-copys to a single piece of 2024 T3 aluminum with spray contact cement (3M brand p/n Super 77). Any where there was a slot I milled away, as a group of course. The aluminum spacer is very light with just enough metal left to support the photo-etched brass decks, but all the slots are open to do their job. This will allow water to flow through all the deck slots. In the aircraft industry this is called box construction and should make the top of the hull very strong.

Good Luck and Good Hunting,
Jerry Pope