Cut simple hole into a cuboid with own CSG

I need to cut a really simple small cuboid out of a large cuboid, preferably using CSG. I looked at many resources and unity packages, but none of them worked for me, or they covered too much I don't even need for this simple task and therefore resulted in really messy things.

That's why I ask if it is possible to take a mesh of an object, put 8 more vertices into that mesh where I want them to be and then there's a square hole in that mesh? Or is it a bit more complicated than that...