I've got a few more MTG EDH decks now than is practical to carry them by hand so I picked up this storage box from Harbor Freight and modeled my own inserts to better accommodate the deck boxes. I also modeled my own deck boxes and have those files available as well. This storage case can hold either 10 decks or 5 decks + some playmats. The slot on the side doubles as space for the playmat or as space to get your fingers in between the decks to grab them. If you have thicker fingers it might still be a squeeze, but it's still better than the original compartments that came with the box. 

You'll need to print 2 of the corner compartments and 10 of the deck+mat compartments to fully fill out one of those storage boxes. I printed everything in PETG so I don't need to worry about leaving the box in a car over the summer as much as if I had printed it out of PLA.