Since I'm sure I'll end up dropping my device if it's not in a case, I bought a ZRANTU Case Huawei P20 Mobile Phone Case with RFID Protection from Amazon to try out.
I thought that with its phone holder portion being rubber, I might be able to get it to stretch in the right ways to hold the Pro1-X, but that was not to be. I ended up tearing the whole rubber portion out, retaining only the wraparound cover, and then stuck the Pro1-X in using some thin double-stick mirror mounting foam tape.
So far it seems fairly good. It's a little tight when closed, but the magnetic clasp does hol