Generate it in any size.
Has an experimental feature to include an extra set of inner walls to protect the hinges from the contents. By drawing the hinge lines in a zig zag pattern, plotting time is reduced significantly.
Change doosje.pl and run it.
Newest version and instructions can be found at http://github.com/Juerd/doosje
Change the parameters for height, length, width and material thickness. These are the inner dimensions, so just measure the thing you want to store inside. The script will output the outer dimensions.
Other customizable parameters include "finger" width and kerf compensation. Read the source code to discover more parameters.
(I didn't know about http://www.thingiverse.com/thing:17240, another parametric flex box, when I made this. There are quite a few differences between these two things since they were developed independently.)