You want 3 things: selectivity (to get the right dwarf, and only them, on demand), simplicity of construction, and (as much as possible), convenience for recovery of items.
That takes anything elaborate such as drowning chambers out, unless you already have them - drainage, grates, etc - too much effort. 3 mechanisms is minimum (one for lever, two to connect it to the object in question) - so let's shoot for that.
If it can serve double duty as defense, so much the better.
Activating the dwarf as military can then < s >tation them anywhere - that's the easy part.
If you have a long drop, then, as suggested above, a retracting bridge over that is the way. If you don't, then digging one is fairly easy. As mentioned, add the spike to the bottom to make sure - that does sound like the best on all points (no pun intended).
If you can put this near your entrance, possibly incorporate it into the path that siegers might take (or that you can force them to take), then it's win/win.
One other way would be to link an existing weapon trap to a mechanism, but that takes it out of the defense (unless you also link it to a pressure plate, but that gets back to being more complicated.) Also the occasional jam, could make other dwarves queezy to clean up.
Other traps, such as drowning or wild beasts, might be more entertaining to construct and employ, but that's personal pref.
(This is a really good idea, btw - no cat lovers, no slackers, no heavy drinkers. Trick is to get to them before they make any friends - which often has already happened on the walk from the map edge to the fortress. Meh - the new migrants should get over it.)