I think you have to make it a little bigger than the spec. A standard quarter barrel keg (or a half barrel) has a circular footprint of 16-1/8 inches, so you’d need the keg box to be at least 32-1/4 inches long. I made mine just barely large enough to accomodate 2 half barrels, but in reality I’ll be using mostly slim quarters or 1/6 barrels because I went with a triple tap.
Also, if you go to a larger keg box, consider using a compact FREEZER, not fridge. AND, add extra fans to keep are circulating, you don’t want to block the opening to the cooling unit.
The new compact freezers get considerably colder than the fridge units.