Boolean Difference on Hollow Object I'm building a submarine that I want to 3d print. I built the hull and now I want to cut a hole in the top so that I can put in the electronics/motors inside and then close it with a lid.

I took a bunch of circles and bridged them from one to another to create an outer skin since I knew the dimensions of the circles.
I selected some faces on the top of the sub that would be the &quot;lid&quot;. I shift-d copied the lid out.
I solidified the sub in order to give the hull some width.
I solidified the lid to give it some width
I do a boolean difference so that I can cut out the lid from the hull.

It doesn't work as expected.
Below are some screenshots where I did the boolean difference on the hull and a long rectangular shape with the expectation that a hole would be punched in the hull. The 1st pic is the outside and you can see the entrance hole at the top. The second image is where it gets strange. Blender wrapped the hollow area that was cut out with a rectangular section.
Some things I checked include whether or not the shape is non-manifold (it is), are the normals pointing off to anywhere weird (they don't seem to be).
What is blender doing here?
I think I figured it out.
I had the normals set facing from the shape upwards. I flipped them on both shapes to face inwards. After that, I did the boolean difference and it worked.