When I put my quarters on, I puposely left big holes in the bottom. When a convertible's top is closed, all of the water coming down the rear roof goes into the rear gutter which flows into the rear window-quarter cavity. Make sure that everything in there is well water proofed too.
I've been caught out in some storms before where the rain was coming down like I was being sprayed by a fire hose. I wanted to be prepared for that much water.