Ernest, not sure if it's too late now or if you really do want to open up the slits in the side of the casing, but I've done similar projects before and found them to be quite solid.
Since you already have the location of the ribs marked by the rivets, you could just cut out the area where the slits are.
Then I'd use an appropriate metal 'rod' and drill or glue it in place between the saddletank and the upper casing at each row of rivets. You should be able to find small brass angles or T's to fit. Although it looks like the edge of a plate, the rivets actually have to go into a flat spot so those ribs have to be either 'L' or 'T' shaped to give a landing for the rivets and the casing and still have a projection inside to give strength to holding the casing in place.
Using several brass or aluminum 'ribs' on each side of the model should be strong enough, especially if they're well drilled and sunk into the saddletank.
Of course, then you might also want to drill out the flooding spaces in the deck too, and complete a section of pressure hull underneath since it would be VERY visible with this method and would make it much easier to show and explain how real U-boats were built.