Good luck getting the black boxes to float!*
Armour plated for crash resistance, for their size, they would have to be the heaviest boxes on the aircraft. You'd have to find a crash resistant/fire proof floatation device of considerable size.
Would the duplicates need to be 'duplicates' as such?
That is would they need to be fully armored (and so heavy) if they were
for ejecting over water surrounded by a floatation device... maybe a compromise
could be made... ?