I posted this on APUG and thought many here might find it helpful:

I've been hoarding plate boxes from my commercial offset printers for shipping mounted/matted prints.

The best I've seen are the kodak boxes (images attached).
Creo and others also have some stout ones, but these are box-in-a-box and there's a one inch space surrounding the interior box.

The one shown has (inside box) a dimension of 22x26. Kodak makes all sizes of plates for the various size of presses. One to fit a 16x20, 18x24...
Contact some of your local printers using sheet fed presses and ask them to save for you.

I must have 20 collected in the last two weeks alone from three of my printers.

Hard to beat Free and good.

Hope this helps..