There were three FBs in the second cavern, having between them noxious secretions, poisonous vapours, and deadly blood. The plan was simple: lure them in with a path to the fortress, then lock the door to slow them down while the bridge raised behind them and the wall was built beyond the door. I was expecting to catch one in each fork, but two of them came in sufficiently close to each other to both get caught.
The design is pretty simple. You have a capture chamber, with a raising bridge and a door. Beyond the door is a wall suspended at the 'nearly done' stage. To make the connection between cavern and fortress you start with an up stair and a natural wall on the level above it. You channel out the wall, making a gap in the floor above, and then construct a wall on the tile with the stairs. This allows you to keep your dwarf out of harm's way instead of having them go through the wall right next to the cavern. With this one I didn't do that completely, as the access tunnel doesn't have its own connection to the fortress. And it wouldn't have mattered anyway, because I had to quickly apply a burrow because there was some animalman stuff that
demanded hauling. Sure guys, wander out into walking typhoon of bodily fluids, I don't mind.
The doors were left unlocked until the tick the FB had passed the bridge tile. Then the door was locked, the wall was unsuspended, and the lever was pulled. Between the emergency burrow and the meeting area on the level above, this happened very quickly. And that was that.