Spark plug tray

I planning to make my own spark plug tray out of some leftover materials. No need to buy one when I don't have to, right? I want to have the tray made before I remove the plugs. I'm using 18mm aircraft plugs.

1. Can someone with a tray measure the holes in it and tell me how big to drill them?

2. Is this hole size from question #1 designed to hold the plugs upright or inverted?

Thanks.

[joeboisselle]

Mine has 3/4" holes (i think) and holds inverted. Just measure the center section of a spark plug, this'll give you how big to make the hole.