No adhesive tape, string or rubber bands, this plastic container has a built in locking system!!!
(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

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

cut vertically down through the mouth and neck of the bottle. carefull, with your fingers.
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

fold the flap outwards and crease it.
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 :)

Step 4: Other Bottles and Plastic Containers

ive tried this with 2litre bottles and a few jerrycans as well. just remember to increse the gap between the cuts(where the fold happens) coz in bigger bottles, it tends to get a bit flimsy if you make it less than 2inches... alternatively you could cut the flap all the way through and glue on a small metal hinge.