I typically do this (which is not minimal):
Bbd.scccBb
where:
B = "joint" of raising drawbridge.
b = "smashing part of drawbridge (to get a graphical indication of whether the bridge is up or down)
d = bait door. Can be locked when needed. The door is expendable (i.e. it will eventually be destroyed)
. = empty space. Can place a cage trap here.
s = menacing spikes (usually 10 green glass) hooked up to a minecart repeater.
c = Cage trap to capture desirable critters, and keep undesirable ones from getting into the fortress.
Normally the outer (right) drawbridge is raised (as I normally don't access the caverns).
When an FB is spotted the menacing spikes are installed (I do that as they're needed, but once done, that's taken care of).
Once the spikes are installed and hooked up, I raise the inner drawbridge, test that the spike trap actually works, and then open the outer one.
Once the FB enters I order the outer drawbridge to close, so it's contained. If it's too tough for the glass spikes I eventually engineer a cave-in (which will destroy the cage traps, spikes and inner drawbridge).
Minimal setup:
dBsB
This carries the risk of the door being destroyed before you close the inner drawbridge
You don't need a drawbridge to keep the FB in place if your bait is an artifact, but anything that can be destroyed will eventually be destroyed, and dorfs have a tendency to ignore urgent tasks in favor of old bone hauling and harvesting (and if you lock them in with the levers they'll probably be asleep when a lever needs to be pulled).