Hi bartosh, your object is not closed after the boolean because the base object you are cutting is not a fully closed solid - it's got a few areas in it where some edges are not joined to their neighboring ones, so it is not forming a completely closed solid volume.

The booleans are mostly oriented around working with solids - when you cut a solid with a surface like you are doing it will leave the imprint of the cutting object behind as well. But when you cut an open surface you'll get different behavior.


It looks like there is a little misalignment between some pieces in this area here, you have to zoom in a ways to see it:





There's kind of a little tiny shelf in there and it's in that area that some of the edges are not joined to one another which is why your base object is not a solid but being treated as an open surface instead. When you select the base object you'll see that it's type indicator in the upper-right area of the properties panel (the place that gives some information about the selected object) reads as "Joined srf" rather than "Solid".

So that part of the object may need to be reconstructed to be all uniform and not have any gaps or things like that, then once you have a solid your boolean difference would leave behind the cutting surface to close off the hole automatically.

- Michael