This is a case for spare spark plugs. It was made for spare plugs for a snowmobile. This case protect the spare plugs and can be put in a glove box. The case has 3 parts, the bottom, top and a plate. The spark plugs screw into the plate and the plate is enclosed between the top and bottom of the case. The plate will secure the plug and keep the plug from rattling inside the case.
The bottom is printed bottom down, the top is printed top side down, and the plate is printed flat. On the plate there is a slight taper in the prongs in the holes, the side with the long end of the taper should be printed down. I included pictures of the top and bottom on the printer bed. No supports are needed.
Screw the plugs into the plate. The plate should fit inside the case top. If it fits tight, scrape the edges with an exacto knife, or file the edge so it fits easily into the case top. The plate will be sandwiched between the case top and bottom when the case is closed.
I made a model of a spark plug based on dimensions found on the internet. I made the case to fit around the model of the plug. I then measured an actual plug for a snowmobile and adjusted the size of the case holes so it should fit about any size plug.
I was considering putting some foam in the top hole to keep the plugs from rattling inside the case. I came up with the plate that the plug screws into. This plate could be designed into the bottom of the case, but I decided to keep it a separate part so it will be easy to remake another if the tabs get wore out or broken off with use.
The 123D design file is included if you wish to make changes.