Huawei Silicon Case
It might not be fancy, have a unique design, or offer a weird dual-layer design, but the Huawei Silicon case is a great option too.
Yes, Huawei makes cases specifically for the Huawei P20 Pro, and this is one of them. You can get an official case from the company in three different colors, and it'll fit your phone like a glove.
This Silicon case isn't the most durable, but it's fairly affordable and will do a good job. Not to mention it's made by Huawei themselves.