When I was a school teacher (grades 5-8) at first I used the 'correct'
word, but after a while I started speaking of 'boxes,' as I intended them
much more than the general case. Many colleagues I spoke with about it had
the same idea. In fact, 'paralelepipedo' is one of the most popular
tongue-twister adults play on young children in Brazil! 'Shoe box' is also
a very common expression among teachers for the 'paralelepipedo
retangulo.'
It would be really interesting to know how the terms fare in Greece, both
in and outside schools.
Hope it helps.
All the best from garden-partying Bristol (UK)--at least for as long as
the Sun is gracefully showing.
romullo (as spelled by my 6-1/2 years-old son, who found 'paralelepipedo'
as easy to say as 'popcorn'--if he had had 10 pints of cider already...