What you are looking for is a NEMA L14-20 flanged inlet.

L for locking, 14 for 120/250 4 wire, 20 for ampacity.

Ask for this at any large supply house, they can order it.

No L14-20 is cheap but certainly you can beat $100.00

All manufactures use the NEMA designations.

A 15 amp duplex at a house would be a NEMA 5-15R.

5 for 120 volt 3 wire, 15 for ampacity and R for receptacle.

Bob Badger
Construction & Maintenance Electrician
Massachusetts