(PS. i hope this simple instructable does'nt already exist...i looked all over the site to make sure)
This is an old DIY i came up with a few years back, to store my paint brushes and other art supplies. Basically i used to ride a bicycle 16km to art school for the first few years i was there. I needed a container that would keep my paintbrushes and paints safe in my back pack.
Step 1: Materials
1 plastic bottle with screw-on lid(thats going to be the locking mechanism)...i found that smaller 500ml bottles are more robust in the long run. However this does work with bigger bottles too, they just tend to wear out faster.
1 hacksaw..or any other tool that will be able to cut vertically down the mouth and upper neck of the bottle(through the threading) where the plastic is the thickest.
1 cutter blade
1 pair of scissors .. if needed for precision cutting
Step 2: Cutting
use the hacksaw to get through the tough part. continue down with the cutter blade or scisors, whichever you are comfortable with.
carefully cut only till about halfway down the bottle...don't over cut. refer to the diagram i've added
Step 3: Creasing and Locking..locking
take a minute to cleanout the insides of any plastic hacksaw dust, and leftover juice/gatorade :)
close flap and screw on the cap to lock your new bottle container :)