I found this simple clear acrylic case on AliExpress meant for a Rock 5 but since it’s open on the front and back it should fit. The best that’s out there for now I think.
I tried looking for one on Amazon and Ebay and could only find cases for the Rock 5B but couldn’t find anything for the RockB+. There doesn’t seem to be much 3rd party support for cases or kits yet but the board is pretty new still.
There are images to 3D print a few Rock 5B cases that have extra room which should support any variations between models.
Rock B Simple Case 3D Printed
If you find something else let me know! Good luck