There was a thread on the ukmiv's recently about this, I can't remember the full thing but there was talk of this same thing. Other than the cut outs for the drivers side I can't see it being that hard to do. I think I would have a separate plate for access to the sump plug, something that just screws on maybe.
Haven't thought it fully through yet but am really keen. My engine bay is manky from living in the sticks so this would be extra welcome and I'm lowered 35 mm which on the Mk4 platform means my undercarriage does take a bit of a knock sometimes. That dip in the sub frame for the dog bone mount has hit off the deck a couple of times too.
Now where to get alloy sheeting and what thickness to use, that's the next question.