Blanking plate for MacBook Pro optical bay?
I have never see anything that covers the area where the slot is, so cannot really help with your query specifically, although I did spend a long time researching the whole caddy thing when I did the same to my iMac, and I've never found anything.

However, I would advise you to blank it off from the inside, with some electrical tape or other, to prevent dust ingress through the slot.

Any external blanking plate, (presumably to remove the temptation for someone to try to use the slot left behind?) is going to be more unsightly that the slot, in my opinion, unless it is perfectly machined out of the same aluminium as your laptop.  I'd be happy to ensure that the opening is secured from dust, and not worry about the aesthetics of a pointless hole, because the alternatives will almost certainly stand out more.