If you are packing a large box or heavy items, consider completely lining the inside of the packing box with sheets of Styrofoam to strengthen it.
Snug fitting foam will reinforce the box and will guarantee that the thickness of the padding will never be less than that of the foam.
Note: You cannot build a good insulated foam shipping box this way. Air leakage will be excessive and the box will not be as well insulated as you would expect.
Styrofoam can be secured to a corrugated box with any number of commercial mastics or adhesives.
2′ X 4′ SHEET (HALF-INCH THICK): $4.50 EACH
2′ X 4′ SHEET (ONE-INCH THICK): $6.50 EACH
2′ X 4′ SHEET (TWO-INCH THICK): $9.50 EACH