Ditto that. Unless it's an industrial unit, it will run fine on a 15A circuit, so it would have a NEMA 5-15P plug (regular 120V plug with parallel blades), which I think you're saying it has. Just use a good quality 15A extension cord.
OR, you could replace the receptacle with a 20A version (NEMA 5-20R), which has the T-slot on the neutral (left side with ground pin at the bottom) and use any cord you want. A 5-20 will accept both 15A and 20A plugs.
The 15A receptacle on a 20A circuit is legal, by the way, as long as there are two or more receptacles on it. Since it's a duplex, it counts as two receptacles. Each one is good to 15A (technically), and your freezer has a 15A plug, so I would just do as blackhat suggested.
Edit: But I definitely NOT cut the molded plug off a new extension cord if I didn't have to. The only difference in the blades of that plug would be the orientation of the neutral blade, so no improvement there anyway. Just the wire gauge would be heavier, so I would source a good quality 15A cord with heavier gauge (like 14 gauge) conductors. My upright deep freezer isn't frost-free, and doesn't have automatic defrost of any kind (heating elements), so it only draws a few amps. A lamp cord would work for that unit. Do check the data plate, also as blackhat suggested.
https://www.amazon.com/Tripp-Lite-Heavy-...A1VHK?th=1