I totally agree with Lc here.....
I too have the same type of valve fitted, same problems with leaves and stuff, and the same solution (bung on bit of cord).
Only difference is that the cord (dayglo yellow - well it was once upon a time) is attached to what I presume is one of the transom lifting eyes on the inside of boat. With the coloured cord, even a casual glance sternwards lets you see when you have forgot to put the feckin' bung in again.
If I ship a lot of water, pull the cord, out comes the bung, and after a little bit of driving around the water has drained, so stop, reach over and put it back in. I suppose it would be easier with an elephants trunk, but the idea of drilling out a bigger hole thru the transom scares me too much - it was bad enuff cutting a hole in the console for a hatch.
Thing is - this type of valve is prone to let water in (albeit slowly ) when the boat is at rest - so I would still recommend using a bung.
"Remember the Plug!"