They're called soda bottle preforms. I'm sure the imaginative ones can come up with other names. As it is in life, they come in different lengths and colours. If they've been mentioned on here before, apologies in advance.
So, they're a thick-walled, hollow cylinder with a rounded end, and they have a threaded top with a solid ring that lives just below the cap to help prevent damage to the cap if it ever gets run over by a dump truck or something. Crank a soft drink cap on that sucker and NOTHING is getting in or out. You can buy them online in batches of 12 or 24 or 36... Drop a TW spark plug into one, electrode down, and the hex body fetches up at the curve at the bottom just before the electrode can hit the end. So the spare plug can't move around, and the electrode can't touch anything. I have one with a spark plug, some electrical wire, some shrink tube, and a few other tidbits in it, and a 2nd one lives eternally where my tool pouch used to be and it contains photocopies of my registration, driver's license, insurance card, and a $20 bill in case I ever get caught without my wallet and need gas and/or food/drink.
They're nearly indestructible, and pretty much leak-proof. I use them for biking and camping, they've held everything from instant coffee to fuel for my trangia burner* to my backpacking fire kit.
Figure out how many you think you want, and order twice as many because you'll use them all.
*make your own decision on using them for flammable liquids. I tested a typical cheap, thin bottle from a case of bottled water and put gasoline in it and let it live there for 6 months. The flimsy bottle and cover showed no signs of weakening or breaking down at all. You're all adults, make your own call there.