Video below: comparison of suction of the Epic V10 Sport (2011 model) with and without a custom bullett.
Watch in HD (720p) on YouTube (just click on the YouTube name in the bottom right of the video below).
I first started the test with a bullet that was a little fatter than the one here. It was still smaller than the vent cover but it still did not work well at all - the suction with the bullet was worse than without it.
Then I reduced the dimensions of the bullet to what you see in the video and now there was some improvement: both in the suction force generated as well as in reduced turbulence behind the scupper vent cover.
I might play a bit more with different shapes yet to see if I get any better than this. As it is, adding this bullet increases the force of suction by about 25% (eyeballing it from the higher water level in the clear tube) and visibly reduces the turbulence (by looking at the thread patterns).
Not sure how this translates to in-water performance: have yet to try it. My first test some weeks ago was in the water with a bullet of size and shape that today I saw did not work at all and actually decreased the suction - on the water that same bullet did exactly the same: reduced the effectiveness of the drain. So I am hopeful that my new shape will work better in the water since it works better in the air
Will see about that soon enough...
There were some other interesting profiles that I think might be tested - for example, adding a sort of "wing" or a "spoiler" over the cover, like on the back of some SUVs or minivans/wagos to direct the flow down and smooth it to increase the local velocity and decrease turbulence.