For prints of this size, I like the method used by West Coast Imaging. Print goes in an archival plastic sleeve with an interleaving sheet covering the face of the print. Sleeve is taped to an oversized corrugated cardboard sheet. One more corrugated sheet goes under the first one, and two sheets go over the print, and then all four sheets are packaged in a corrugated cardboard box. Any additional materials or documents can go between two of the other sheets of cardboard. It's a very sturdy package.