joining vertexes into faces

[engrenage]

I am trying to make a box for some electronic circuit.

I found out what "intrude" does when a face is selected, and that is really cool.

now want to add a hole for a slide switch ; one both sides of my box I used the "inset" tool, cut a hole there but now my object is not a solid anymore.

since my hole is basically a rectangle prism, can I select 4 vertices and "fill" the space in between with a face, and repeat for the remaining 3 faces?

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a question that is probably similar is, in general, how to "drill" a hole in a model (preferably normal to a face, but not necessarily)