In August 2018, Nokia Mobile launched a new type of protective cover called Nokia Entertainment Flip Cover for low budget Nokia 2.1. This flip cover is now launched for Nokia 5.1. Plus according to the Nokia’s international accessories webpage.
The design of the cover remained the same, so did the color options. You can choose between black and cream color of the cover. The cover is made of polycarbonate and is a bit heavier (132.83g) than the one for Nokia 2.1 which is made from lighter polyurethane. What worries me the most is that the cover thickness is 24 mm which is going to make your 9 mm thin Nokia 5.1 Plus fat. The reason behind that gotta be in providing the better protection for glass covered body of 5.1 Plus.
The price or the availability of the cover are currently not known. For more info check the official Nokia page.