This is along the lines of what I was thinking, but given that I just paid like $800 for this skimmer, I'm not really wanting to permanently change the cup. Also, I like your design better for the lid, because you have a longer acrylic piece that directs the skimate down. Mine is very short, and I feel like skimate would get in the barbed fitting. I regularly have skimate escape though the holes on the top of the lid.
I actually figured out that Reef Octopus sells a replacement lid for a reasonable price that adds this attachment, but it's out of stock everywhere
Whiskey