I was searching for protective spark plug carrying cases for use when dirtbiking with my kids. Since we have 4 bikes, I needed a variety of sizes for all the different plugs. Many of the designs out there were either too bulky or didn't fit correctly for various plugs or configurations. I took inspiration from user Nick_Groot on the top cap design with the grip surface so you could use the top to start a plug into an engine.
I designed them with a positive snap system to keep them closed. This was the hardest part for me. I originally had the snap feature running all the way around the top part, but it made them really hard to open. I finally thought of the idea of limiting it to two small tabs and then adjusted the size of those tabs until the effort required to open and close them felt good. I designed them to accommodate both short and long thread models of plugs, with or without the top screw-on aluminum plug cap. There is a small rectangular pad provided on the back side for labeling the plugs by either the plug part number or the bike it fits in for convenience.
I am also including the Fusion360 models should you need to make modifications.
I suggest using brim build plate adhesion since the footprint is a bit small in the orientation they print. No supports required. Depending on your print resolution and accuracy, you may have to clean up the little bead area on the snap features a bit to make them perfect. Mine always seemed to have a small lump or two that I had to clean up, but other than that, they seem to work well for me.