To add to that post above, I also lined my boxes with some 3mm thick hardboard, cut to fit the insides of the box. It didnt give much 'extra' strength as such, but does stop the box being damaged or the things inside being damaged just in case something happens. The hardboard was only something like £3.50 for an 8' x 4' sheet (needed 2 sheets) in B&Q and they cut to size on their gurt cutting machine.
