There are many ways..
One would be to put the items in a family - a create family would take a random item from the family
Another way would be having a variable and setting it to choose(1,2,3,4,5) if all of them have equal oppurtunity to be created
So:
on box destroyed
set variable to choose(1,2,3,4,5)
--- if variable = 1
create item 1
--- if variable = 2
create item 2
--- if variable = 3
create item 3
etcetera
-- variable