Hard, you have one too many ring gaskets, there.
I've used the one Jimmy is noting, but they always seem to catch floating weeds and get fouled up. I got tired of always reaching overboard and cleaning them so I went back to the normal style and have been happier. Yes, if a couple guys are in the back of the boat at the same time some water may back up into the boat - but as soon as we move, the water goes right back overboard. Those scuppers are not meant to prevent that. They're meant to prevent large influxes of water, which they do quite well. Besides, if you start to get any growth inside that clear plastic cup, the ball won't seal well anyways, and the same thing happens. It's a nice design, but over the years I've found the standard scupper to be the better, overall solution.