What about this???

[bigeasy1]

I've pondered the same question,and searched and searched all the scupper questions on this site and others,but with no luck.
I think the flappers are made to work better when semi horizontal,not straight vertical.

When they are in a somewhat vertical position,the rubber flapper,hangs fairly straight,and has a natural tendency to easily swing closed with a slight amount of back pressure from the water.

If you place it on a vertical plane,it's going to always want to hang open.I,m thinking that if it opens too much. water pressure will not be acting on the face of the flapper,but only on the edge,making it difficult to work the way it's designed too.
Of course like many things I've thought,I could be completely wrong.

I'm going to try using the VORTEX thingy,I tried them out on a two liter soda bottle,and they work great.

How much water enters your boat through the floor scuppers?I have the same exact boat as you,and haven't splashed it yet so i have no experience with the water problem.