This is a remix from @TheGuy on printables. (https://www.printables.com/model/422078-subaru-sparkplug-holder)

Original model had a space in the middle and I filled the area out to give it more strength. I also modeled the numbers so you can print them in a contrasting color as more of a visual aid. 

The numbers are sized for a very tight press fit. Play with the sizing to get the fit that's easiest for you. Also, the number are on a separate plate because I recommend slowing down your speed when printing the numbers to greatly improve the quality of the edges as they are small