Hi,

I usually use moving boxes, or some other thicker 
cardboard.Cut out the requiered size for top and 
bottom and tape them together,make sure to put 
the cels in extra bags (if they're not already) 
so they won't stick to the tape you used to put 
the cardboard together. Of course, you can fix 
the position of the celbags then with tape on one 
of the cardboard pieces, so they are always in 
the right position and won'tmove in the package. 
And that's almost it, if it's still bendable, put 
some more cardboard inside.
hope that helps a little
Seb