The box wouldn’t look like a frustrum, as it’s a box in the physics world and not in the camera perspective.
If you just make a box with the upper left corner where you started dragging, and the lower right corner where you stopped dragging, then you would get a box in the world.
If you multi box trace this with a filter on the actor class of your unit type, then you would just directly get all “Units” from this query without needing to do your current “selection logic”