Do you have access to large pieces of old cardboard? If you do just cut it 4 inches wider on each length than the dimensions of the mounted piece. ie for 16x20 do 20x24. This gives you a good 2 inches on each side. then sandwich the work that has been covered by two pieces of archival paper between 3 to 4 layers of cardboard on each side. Use duct tape to cover all ends and sides. This gives you a stiff and well cushioned container that holds the work flat and safe. (I love duct tape)
you can go to your local large chai electronis store for the cardboard. They usually just throw it away. I bet there are other stores that have it also. I am a great one for using free things when I can get them.