There is nothing wrong with simple, especially in today’s world where often the complex is used when something basic would do. The El Gringo Wall Mounted Surfboard Rack is about as simple can get. It is made of steel or stainless steel with cork pads to protect the surfboard and it is being produced by Make by Consult, which previously made a similar bike rack.
At $125 it isn’t exactly cheap, but it probably beats trying to make your own.