For a 16-inch pizza, a flat-bottomed delivery-style thermal bag or a square-profile commercial delivery box would be a more reliable transport solution.
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By vevor
Because of the internal structural ribs and the curve of the shell, a 16-inch square box will likely get stuck against the sides before reaching the bottom.
You may be able to fit the box if it is tilted at an angle, but this would prevent the lid from closing and could cause the pizza toppings to slide.
By vevor
A standard 16-inch pizza box typically measures approximately 16 x 16 x 2 inches, while the interior floor of this 48L case is slightly narrower due to its aerodynamic, rounded shape.
The external dimensions of the case are roughly 21.3 x 17.1 inches, but the internal usable width tapers significantly toward the hinge and the latch mechanism.
By vevor