Boolean Difference Manipulation

I had realised there were 4 vents in each side piece of the Star Destroyer and wanted in semi-circular edge or cut into the object but was not sure how to do so.

My first attempt was to place a cylinder and then a cube inside of that where I would rotate the cube to cut away more than just from top to bottom but across the edge. I found this did not work at all and left straight edges inside that looked very messy.

My second attempt was to replace the cube with another cylinder so the other cut was rounded also. The end product left an almost figure '8' shape across the object and looked messy once again.

My third and final attempt was a success. Rather than using 2 object to cut, i would just manipulate the single one. By selecting a semicircle in the top face and translating it across it created the perfect effect for what i wanted to but cut out of my object.