Why do we say "He will pay her for an extra two hours of work" when "two hours" is obviously plural?" "I'll take an extra dozen." "It will accommodate an additional three persons."
In this kind of construction, we are thinking of 'two hours of work' as a single 'amount of work', and three persons' as 'a single group of people'. That's why we say 'an extra two hours of work' rather than 'two extra hours of work'.
The word 'dozen' is a singular noun. We can count dozens.