The idea for this set of things it to make the base designs for custom outlet covers both decora and duplex.
The first goal of this project is for electric safety for families with young children with an unhealthy interest in electric outlets. Some of the covers have the electric holes completely concealed -- if you know you are never going to use them - this is a nice semi-permanent way of eliminating electric hazards without the hassle of dealing with the plugs. In addition, you can print out as many plugs as you need for your existing outlet covers. These plugs are designed to fit snug into the covers. They also have a nice fingernail groove on the top and bottom of the back to make it easier for adults to get them out.
The second design goal was ensuring a tight fit between the covers and the safety plugs to improve energy efficiency. The energy auditors in Ontario are trained to tell homeowners to seal off holes around outlet covers on both external an internal electric outlets - as major heat loss locations. I have not quantified this yet - and I suspect it is a relatively minor loss -- but these covers will provide some additional reduction in air infiltration around outlets. For outlets that you know you will never need to get into you can permanently seal around the cover or paint over the edges.
"Fancy" houses sometimes have customized outlet covers that add to the look of a room and cost a small fortune. Now with this set of things, some easy mashups with cool-looking designs already on thingiverse and a bit of paint you can make your outlet covers as classy or classless as you like.