Sand clock creeper farm

Creeper farm built on the ground with a water flush, no cats and no golems. Creepers spawn in a room exactly two blocks high, trapdoors under the whole roof keep zombies, skeletons and endermen out because the ceiling is too low for them, and a piston and sand clock releases water that pushes the creepers into campfires in the corners. You AFK on a scaffolding tower above it. The campfires are also what caps the rate, because a creeper takes about twenty seconds to die on one. It stacks, but the AFK tower stays at the same height no matter how many layers you add, or the bottom layer falls out of spawn range.

The item list on screen is incomplete. The slabs are missing and so is everything for the redstone clock, and several builders had to go back for materials halfway through.
The water breaks the redstone when the sand does not fall in time. Use sticky pistons instead of normal ones.
Add a redstone repeater every nine blocks or the pistons never fire. One builder says nothing worked at all until he did that.
Keep the spawn room exactly two blocks high with trapdoors under the entire roof, otherwise zombies, skeletons and endermen spawn in it as well.
Extra layers are fine, but do not raise the AFK tower for them, or the bottom layer drops out of spawn range.
Box in the top of the AFK tower so phantoms cannot reach you while you stand there.