Thats a pretty cheesy setup. I think what I'd do with that is create a whole new mount... I went out and snapped a picture of a 25" Dynamo mount, which I think is very cool setup and cheap to make. The pic is of the monitor mount itself, made of 3/4" ply. You make a square board with a cutout around the screen (which is naturally NOT square). This part fits into another piece of 3/4" ply hard mounted to the cab side. Since the outside is square, it fits flipped either way.
The cab that frame is hanging on has a similar setup, but even has oval holes in the part around the monitor to act as grab handles. Its a bit of measuring and cutting initially, but I can swap monitor orientation in about 5 minutes.