Slanted Clipping in Hammer

How do you clip at a diagonal downward on an object that is diagonal to the grid.

For example in this old picture the bar that runs along the bottom of the tanks and around the room is slanted diagonal to what would be the grid in hammer sometimes but also has slanted cuts up and down. How do you do this since the object is not aligned with the grid you can’t just make a normal diagonal cut.

Founded in 2004, Leakfree.org became one of the first online communities dedicated to Valve’s Source engine development. It is more famously known for the formation of Black Mesa: Source under the 'Leakfree Modification Team' handle in September 2004.