1) Long corridor (mine is about 100 tiles long, 3 wide; I put no research into timing, I just made a really long corridor).
2) Raiseable bridges at either end of corridor. I don't have much experience with building destroyers, but I do believe that trolls will wreck your fun. Sorry.
3) Pumps along one side of the wall, pointing into the corridor (one every 10 squares). Pumps are sourced by an aquifer (convenient).
4) Raiseable bridges along the other side of the wall, beyond which are grates above the aquifer. This'll be the sink.
5) 3 pressure plates in the middle of the corridor (going across).These are the on switches; they're triggered by non-citizen creatures size 4 or more. I call these pressure plates A.
6) Two pressure plates, one at either end of the corridor. These are the triggered by 7/7 water. These are pressure plates B.
So it looks like this:
XXX...XXX
XXXbbbXXX
XXXbbbXXX
XXX..^XXX
XXX...XXX
XXX...bbG
~pP...bbG
XXX...bbG
XXX...XXX
(pump pattern repeats)
XXX...bbG
~pP^^^bbG
XXX...bbG
(more repeats)
XXX...XXX
XXX...bbG
~pP...bbG
XXX...bbG
XXX...XXX
XXX..^XXX
XXXbbbXXX
XXXbbbXXX
Elsewhere, the control mechanism, shown here in vertical, not horizontal, mode:
X~~~XXXXX
XXXcXXXXX
XXXc__ XX
XXX^FF X
XXXXXXX X
X = wall
~ = water
^ = pressure plate
c = hatch cover
F = floodgate
The "on" pressure plates from the corridor open the top hatch cover, allowing water to flow down.
The bottom hatch cover is controlled by a lever; if it is open, the trap is "armed"; if it is closed, the water will drain off harmlessl and the trap is "disabled".
If both hatch covers are open, water lands on the pressure plate. This is pressure plate C, and is set for 2/7 water or higher. When it is tripped, the bridges in the corridor are raised, and power is supplied to the pumps. Result: corridor begins to flood. This is setup like this so that once pressure plates A have fired, they're not required for further flooding of the corridor -- it doesn't matter where the creatures go now, or if they do or do not step on the "A" pressure plates.
When the corridor has flooded almost completely, the "B" pressure plates in the corridor will trigger, responding to the 7/7 water. Each of these is linked to one of the floodgates in the control mechanism, so they both have to be "on" for water to drain off of pressure plate "C". When they are both open, and water has drained off so "C" is at 0/7 or 1/7 water, it'll trigger again, lowering the bridges and stopping the pumps. Water then flows through the grates on the east hand side into the aquifer.
(Grates are installed to keep the loot from flowing into the aquifer too).
So the timing is controlled by how fast water drains off of the "C" plate. In the setup provided above, this turns out to be just slightly longer than it takes a goblin to drown. To increase the time, just make a larger room for the "C" pressure plate, so more water will fall in from the hatches.