Thanks! I do a box trace from the camera plane with the size of the blue box until I reach the Ortho Far Clip Plane distance.
The only issue I have is that this also detects the floor. With a MultiBoxTrace, I also get the black box. Not quite sure how to handle this. Maybe I have to count if the number of objects hit by the trace changed.
EDIT: Seems to work when I check for initial overlap. Now I need to find a good way to block movement like the built-in collision system would.