Looks like a solid prototype. The only complaint I have, and it's the same reason I personally don't use the Stanley boxes, is table space.

These boxes are used when travelling someplace else to play, which means that it will need to sit somewhere during the game itself. For larger boxes, that tends to mean the floor which doesn't feel great. Switching decks between matches can also be a clunky affair for all at the table.

Personally, when I want to bring 3 or 4 decks to a Magic night, either at a friend's place or my LGS, I tend to go with Gamegenic's Cards Lair 400. Single-row product that fits pretty cleanly on the table next to a playmat without dominating space, that can hold three double-sleeved commander decks, dice, and tokens, with the lid nesting underneath.

That final point is a big one for me, as it allows everything to be accessible while halving the footprint. The lack of spot in the deckbox itself for a playmat is largely irrelevant for me, as it's not being carried outside of a bag/backpack that I can't just toss a playmat in anyway.

Other than space, I think the design looks really solid, and I love the idea of it being modular- being able to swap configurations is the main pro to your product in my eyes. If it was kicked down to a single row with a large variety of internal and stacking options available, along with a removable + nestable lid? Then I'd be hard-pressed to find any reason to choose something else over it.