Thing is, you can do this even with ordinary furniture (although you have to watch it closely lest the beast wander away). When I crack open the caverns, i create a second entrance that leads only to furniture. If a FB or spider is spotted, I close the gate that leads to the fort and open the one that leads to furniture. It might be laid out as follows:
############
#XX........#
#XX........#
#########..#
........#..#
........#..#
###D#D#D#..#
###D#D#D#..# D = drawbridge
###.#.#.#..#
###.#.#.#..#
###F#F#F#..# F = furniture
###F#F#F#..#
#########..#
...........#
...........# <- dwarf entrance to caverns
############
In this mockup the cavern floor is to the west. Dwarves take the long way around (east>south>west). When a FB or other building destroyer wanders into one of the furniture-display cubbyholes, you simply close the drawbridge and trap it forever. The nice thing about this layout is that if you have trapped a spider or webbing FB, you can use their webs to secure the main cavern entrance. Build cage traps to the south of their prison cells, hook up a bait animal, install a drawbridge to turn them on/off, and carve a fortification into their cell wall. They will web the cage traps, guaranteeing capture of any non-webbing enemies (even FBs) that try to enter through the dwarfs' entrance.