A company called Backpack Shield is manufacturing Level IIIA (9mm FMJ, .44 Magnum SWC) ballistic plates designed to fit into typical high school and collegiate backpacks. The company says that these are carried by Federal Air Marshalls.

I can see the benefit of these shields for businessmen and woman travelling in unsafe countries but unable or unwilling, for whatever reason, to wear a ballistic vest

The plate is 17” high x 12 wide” x .375” thick and weighs 2 lbs. It can be purchased for $235 from LaRue Tactical.