Shotpeened,
The skymarshals are more expensive than the seperate cockpits.
You got to block a seat for the skymarshal. Then you have to pay him to sit in the seat. All the time.
The ElAL apparently allready use my method of a completely isolated cockpit and that is most likely gonna be the way to go. On the A300 that I fly, it can easily be done with no loss of doors or seats, though an extra door to the cockpit would have to be added. But adding a door is no big deal. BIG doors are added to aircraft all the time when they are converted to freighters.
This will happen again, if only because it was so wildly successful.
The answer in the short term is sky marshals, untill the aircraft can be modified. Then once the aircraft is modified, then the skymarshals can be removed.
Cheers
Wino