Just wanted to share this with others whom must park their Mooney's outside. The previous owner warned me that water can migrate into fuel tanks so I made this magnetic rain cap. To be honest I copied the idea from an AME on the same field as me. In his case he used a tupperware lid. I couldn't find the same lid so I made my own.
I vacuum formed thin polypro over a ceramic magnet purchased at Home Depot. Then I stacked the remaining magnets on the vacuumed magnet. The seal is glued to the plastic. It seems to hold well. I plan to wrap some black tape around the exposed magnets so they are not knocked off when I throw the rain cap in the back of the plane.