How can I modify my grinder to harvest endermen?

I've constructed a simple canal-based grinder in the same configuration below:

XXXXXXXOOXXXXXXX
XXXXXXXOOXXXXXXX
XXXXXXXOOXXXXXXX
XXXXXXXOOXXXXXXX
XXXXXXXOOXXXXXXX
XXXXXXXOOXXXXXXX
XXXXXXXOOXXXXXXX
OOOOOOO  OOOOOOO
OOOOOOO  OOOOOOO
XXXXXXXOOXXXXXXX
XXXXXXXOOXXXXXXX
XXXXXXXOOXXXXXXX
XXXXXXXOOXXXXXXX
XXXXXXXOOXXXXXXX
XXXXXXXOOXXXXXXX
XXXXXXXOOXXXXXXX

X = cobble O = water

I enclose this in a cobble wall all the way around and cover it too. There are 3 empty blocks above the spawning areas in each corner.

This works great for mobs apart from endermen. I never get a single one. I know it is high enough to enable them to spawn but according to the wiki, they will teleport when they come into contact with water unless they are on fire. I suspect they are teleporting out to somewhere although I do not know where!

What can I do to ensure they are spawning and can anyone suggest how to get them moved into the centre 'drop' hole?

One way of catching mobs without using water to channel them is a dropper:

X: Cobblestone
S: Sign

SSSSSSSSSSSSSSSSSSS
SXSXSXSXSXSXSXSXSXS
SSSSSSSSSSSSSSSSSSS
SXSXSXSXSXSXSXSXSXS
SSSSSSSSSSSSSSSSSSS
SXSXSXSXSXSXSXSXSXS
SSSSSSSSSSSSSSSSSSS

3 blocks above this cover the whole thing in a layer of cobblestone so that the spawn area is dark. Light the top of the cobblestone to prevent spawning there.

Build your trap a sufficient distance above the ground such that falling mobs will instantly die. Light the ground too. You can build it so that mobs fall into water streams (but still die instantly) so that their items are collected.

This trap takes advantage of the fact mobs treat signs as solid blocks, so will walk straight off a high drop and fall to their deaths. Since it involves no water (pre-murder), it will catch endermen.