I like using a much more standard type of magma trap, but its a multi-part trap. The magma is merely the last step.
First up is a pair of bridges, offset to each other, all linked to the same lever or repeater (which can then be turned on or off by lever). One bridge will always be up, the other will always be withdrawn, and it alternates.
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X.1.XXXX.2.X
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At all times there will be an entrance into the fort, but it will alternate between bridge 1 and 2 continually.
Below this is a pit. Not a very deep one, just a single level drop. Its to "collect" things. Elves, goblins, whatever.
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X..........X
X..........X
X..........X
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A retractable bridge is the floor of this holding pen. Once I've gathered up an entire siege or caravan, I then dump the holding pen to the lower level.
Depending on my mood, its either a giant water powered flusher, using pressurized water to push them off into a splattering pit where everything can be collected.
I could also put random spike traps in the lower pit, or simply flood the lower pit with magma.
I also occasionally use them as part of an arena. Once everything is gathered up I'll park my army on the other side of the entryway to the lower pit and open the door, letting my military slaughter everything.
Its a very flexible design, and can easily catch even trap immune or building destroyer creatures, and then once you catch them in the holding pen you can subject them to whatever fate you want.
I do personally prefer water instead of magma actually, just so that I can use the water to flush them into the deep pit where they explode upon impact. Each body part then decays into a bone, and so I can decorate more things with bone.