I am also looking. According to google, the only center caps that fit RPF1s are the ones that come with them or some stock model Honda caps. I have found DIY stuff all over but nothing that just fits. I just bought (4) 2" knockout closures from the electrical department at Lowes/Home Depot and they fit great. I am planning on carbon fiber wrapping them then putting on the stickers I found on ebay (shown below) on top of that.
Here is what I bought from home depot:
Here is the center cap sticker I am putting on it:
