I was looking for a keyhole insert replacement for my antique cabinet. All I found on the market was a larger version of it, so I decided to print it.  The outer dimension of the keyhole is: 

W: 10mm

H: 16.7mm

D: 4.6mm 

with slightly smaller W and H towards the end of the keyhole to make it easier to fit into the furniture.

I suggest printing it in brass like PLA/PETG with a silk finish to match the old brass look.

The original metal-made keyhole is next to the printed part in the first (cover) photo.

With a bit of polishing paste, you should be able to match the look of old brass.