These quick-to-make box closures keep boxes closed, and take about 5 seconds to close or open.
(Granted, tape works well, but if you use the things stored in a box very often, it just takes too long to open or close a box. Putting something on top of the box works too, though does make it annoying to get to other boxes underneath the top one...)
1) A thin strip of something easy to cut.
For the demo, I used a 1/4" (6mm) thick piece of wood I had laying around, but there's no need for it to be that thick - in fact, somewhat thinner (down to about 1/8") is better - so you have lots of options... paint stir sticks, shims, bits of plastic stock, those cheap wooden rulers...
2) A thin strip of metal.
I used pallet banding, but you could probably use strips cut from steel cans, or a bit of thin spring steel.
3) A saw, to cut make a slot in the wooden strip and cut it to length.
4) Snips, to cut the metal.
Step 1: Cut Materials
1) Mark and cut the wooden strip to a length 1-2 inches shorter than the width of the box.
2) Make a slit in the center of the wooden strip about half-way through it's width.
3) Cut a pair of metal strips to a length of about 2-3 inches.
Step 2: Bending & Assembly
I haven't made very many of these, but the best way seems to be to put the strips in the slit first, and then bend the protruding tabs.
However, it also works to bend the pieces of metal around a pin - which must be thicker than the piece of wood - and then assemble the whole thing.
In either case, you have a lot of flexibility with regard to the angle you bend the tabs to; your closures should work well so long as the tabs form an angle with the wooden strip of something like 60 to 20 degrees.
(As a note to the precise: only one of the tabs actually affects the difficulty of use of the closure; this tab can be made longer and at a shallow angles for a more secure closure, or short and at a steep angle to facilitate use. The other tab should be long and at a shallow angle.)
Step 3: Use
1) With the box open, place one end of the wooden part of the closure under the flap of the box.
2) Slide the closure onto the flap of the box until the metal tab secures it there.
3) Gently pull the sides of the box apart, close both flaps, and then release the sides of the box.