There are plenty of inserts for dominion, but neither seems to be optimized for the original boxes in which the game and it's various expansions are distributed, so I've designed my own.
Since the game boxes aren't exactly three times the length of a card, I've designed the insert to hold the cards in slightly tilted manner - which has several benefits:
I'm able to get three big expansions (or two big and two small ones) into a single box this way. For example I have the following expansions in just two game boxes:
For a single box you need three middle pieces and six sides.
I've used the excellent Dominion Divider Generator to print the card separators (on a normal paper printer :-)).
The author marked this model as their own original creation.