Unfortunately, FB's can't be made of adamantium or slade... unless you mod in a dupe to let them be. =D
The main goal a cavern entrance is progressively heavier defenses with shutoffs everywhere along the line. Due to possible epidemics you need everything shut off from the main fort and controlled by airlocks. Anything that gets a syndrome bearing substance like dust or blood needs it washed off and/or be cleansed with magma so you need a decontamination shower. Go ahead and fiddle with designs until you find something that works for you. A 3 wide and 5 deep room has a good success rate of getting all the dwarves in when stationed. Then have a depressurized water source empty to the side and flow past bars to wash the contaminants off. Check all the dwarves afterwards to make sure nothing remains. Open the shower back up and let them out. Check them again for any contaminants. Then open the bridge to the hospital/dining room/cav. op. dorm and let them rest up. The sewers handle your water/contaminant drainage so flooding will never be an issue. Use raised bridges all along it to control access, drainage, and service access. A Sealed fort is a healthy fort!
@Celem sorry, it gets hard to show it on a cross-section when it all overlaps and top-downs are a bitch to align properly. Once your dwarves reach the quarantine platform on the left, say a 5x5 floor, on one side you need a raised bridge airlock to a show and on the other side a raised bridge airlock to the hospital/living quarters. 3rd side goes through multiple airlocks to get to the main fort. Sewers are clog-proof(at least 2-z high where water will flow) drainage planned to be immune to building destroyers with service access via raised bridge airlocked service tunnels in case a croc survives a fall and you gotta go clean it. (Never forget the raising bridges to seal off the drainages fortifications. You may have to purge some contaminants with magma to keep it clean down there. Or toss PoW's down there and watch the syndromes consume them see if I care!)
As an addendum on cavern breach entrances, Everything needs to be cleans-able with water and then possibly magma to prevent any possible epidemic from starting. Sewers should always be shut-off-able for when you need to flood and keep the entrance flooded. You can put an airlock filled with repeating masterwork steel spikes in the trap corridor leading into the cavern. The cave-in is your last resort before having to obsidianize the tunnel to the cavern so make it long and have a couple burrowed leverdwarves.[Yes... setting those profile settings is gonna suck]
Also, this design lacks any plans for FB zoo capturing as I've never attempted that and therefore have no advice.