I use the same style mount for my GPS when its raining. All I did was unscrew the mount on the rear of the gps holder. It's held on by a couple of screws. Then I replaced it with a ram ball. It's held on by two screws also but it has just a little larger spacing so I had to drill two new holes. I sealed it up with some waterproof sealant and it works fine.
I use my normal Ram GPS mount on dry days and the swap over to the waterproof soft box on wet days. If I remember correctly the new ram ball was only $5 to $10 off of Ebay.
It's kind of strange, how the mountains pay us no attention at ll. You laugh or cry and the wind just keeps on blowing.
Flint Gray 2002 DL1000