The listing issue is interesting. I looked at a bunch of covers and there seems to be a mix of listed and unlisted. The normal commodity plates, made by major manufacturers bear a listing mark on the package but there are plenty of "designer" plates that do not.
As an inspector, if it looks OK (sturdy. reasonable sealing and no combustible material facing the inside of the box) I would not give it a second thought. I actually had to look hard to see a "U/L stamp" on a Leviton but it is there.
