OK i have found several of these and boxed them in and made drowners with glass walls so they come towards me and then drown and give up their loot. It's like a vending machine.
The only thing you have to be careful of is make sure the area on the other side of the glass is doored or bricked off and well lit so you don't have any surprise visitors on your side.
This one is 2 blocks wide for spiders, not sure if they would fall down a single brick water collumn but for skeletons and zombies you can make it one brick wide. I havn't found a creeper spawner and i don't even know if they exist but if i did find one it would be a prize.
Having the flowing water in the room helps move them into the drowner. I am not sure if you have to keep the spawner above water but i do anyway.
So you see how it works, the monsters get in the water and with all the water pouring on them they drown and then the loot washes out the sluice at the bottom for you to pick up.
Beware spiders fit under the sluice but can't get up through the single brick space but they can get you when you drop into the sluice to get the loot.
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Red = Spawner
Indigo = Water Block
Turquoise = Flowing Water
Light Turquoise = Glass
Light Yellow = Ladder
Grey 50% = Coblestone
White = Air