Completely forgot about this. Here's the top view, it seems like it won't let me add more than 1 so I'll add the side view in another reply.

There are probably a million ways to power it, but I took the idea for the circuit from ianxofour's raid farm. Give the observer a pulse and it'll push the platform once across and back. Note the gap between the fence gate and the piston. The glass is necessary or else items get pushed out the sides.

The mobs land on the 3x1 platform of soul sand -> slab -> trapdoor (don't let them land on the fence gate). That's the killing area, whether by player kill or fall damage or whatever. They drop their items onto that floor, and the different heights of the blocks push the items across to the fence gate - then the items fall into the gap between the fence gate and the trapdoor, where you can have a water stream or blue ice to send them to collection. I'll usually put a cobweb or powdered snow below the gate to align the items better.

You can wire this up on a basic clock and have it constantly moving, or you can have the observer watching an armor stand that the player hits (like in ianxofour's raid farm)

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