How to get a box cast to fit around the GameObject?

So I've been trying to set up a cube (GameObject) and do a boxCast from it but I want the boxcast to fit around the cube regardless of the size or location. Everything I do seems to always either result in one of the following:

• origin set to centre or outside of box

• size of box exceeds the size of the cube

• even when the origin is set to top of the cube the box only seems to go half way down

This seems like one of those things that should be pretty straight forward so I'm not sure why I'm struggling with it. Or Maybe I've misunderstood how a box cast actually works.

I would truly appreciate if somebody would break it down to me and explain how this thing works and how I can achieve getting the boxcast to fit around the cube.