Spark Plug Protective Cases

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 have provided these 3:

14mm thread - 21mm hex
12mm thread - 18mm hex
10mm thread - 16mm hex

These will accommodate all of the sizes I had.

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