I've got an embark with 9 dragons in a cave and I want to trap them all.
Holy sh.... you're a lucky bastard!
Regarding the cave-in method, lots of FB-s are completely immune to be knocked out. So I prefer webbing. The big downside of this method is the citizens can (and will) disarm a whole webbed trap array built in dozens of hard work in moments. My hint is to use doors to keep dwarves out of the trap area.
Here's my FB trap design:
██B=> Path to the caverns
██B█████████ <- Blast door (raising brige)
██^╬^^^╬╬ ^█ <- Cage trap to re-capture GCS when opening maintenance door
██^╬^╬^╬+╬█+ <- Door to release GCS (Closed, just unforbidden it to capture her)
██^╬^╬^╬p╬S█ <- Chained (p)ig enclosed with doors on both sides, and an untamed Giant Cave (S)pider behind
██^╬^╬^╬+╬██
██^^^╬^+ ███ <- Door to prevent dwarves to wander in the trap hallway (Open)
████████+████
████████☺████
████████ => Path to the fort/delicious FB-food (Another blast door is preferred here.)
Note that the spider can shoot only three rows far... by the way, it's still enough to catch approximately a dozen+ monsters. If you can separate your wavers, you can use them to disarm the traps by collecting the silk sticked on the cages. (They are immune to webbed traps if they are collecting webs.)
Spoiler ahead!
Allways close the cavern entrance and never forget to forbid the armed cages before disarming the array!!! ...or just figure it out yourself, have fun.
I'm thinking about starting an SCP themed community game, the goal would be building a facility to capture and experiment with forgotten beasts and other nasties... would you join a game like that?