This adapter is designed to work with a bare hard-wired socket you would find at a hardware store like Lowes/Home Depot.
This is the socket I used: http://www.lowes.com/pd_75121-37672-884413___?productId=3379428
Print both inner and outer parts. You'll need support on the inner part.
The author marked this model as their own original creation. Imported from Thingiverse.