I finally found it! A replacement splash pan! YES!!!
These are no longer made but they fit my Shimpo RK-2 wheel. I don't know why they aren't made anymore. 2 piece splash pan that just slides right on (no taking the wheel head off) and the connecting piece has a shelf.
I purchased the shelf portion about a year ago when it randomly showed up at Seattle Pottery Supply, but a corner of it was broken off. So when I walked in a few days ago and they had both pieces in pristine condition, it was like a light came down upon it and angels sang!
The guy said that it's the best condition they've seen one of these in for over 15 years. And I got it!