The most unreliable way of testing how random are scrapboxes: Opening an infinite stack of them in front of a transposer connected to a diamond chest (Iron Chests is awesome):
As you see, I got 18 diamonds, 16 pumpkins, 12 slimeballs, 21 grass and other stuff.
(by the way, I saw IC2's source once - the scrapbox randomizer is NOT weighted)
Now to automate the system...