This is a replica of the safety latch by EliteNetworks. I eliminated the corner brace so that it will print well without support. I also countersunk the screw holes so it works well with standard wood screws.
Update: I rounded off the sharp tip because it was too poky. I also added a fillet in the corner to relieve the stress and make it less prone to break off. New version is mylock3.scad.
Print it out and mount it to the inside of your cabinet doors using #6 x 3/4 inch wood screws with countersink heads. Sorry, I don't know the metric equivalent of this screw. This should keep the little kiddies out of your cupboards... until they learn how to bend it out of the way.