Silicone earloop adjusters, also known as cord locks. Thread these on to your mask's ear loops and slide them up to make the loops the correct size for your ears. Prevents ending up with ear loops that are too tight and pull on your ears, or too loose and won't stay in place. Easy to install with a needle and thread to pull on to the elastic loop. You can also use a fine crochet hook.
We use these with our popular Rozcraft facemasks with elastics up to 8mm wide. The small hole in the adjuster is less than 2mm in diameter so can be used on thin elastics too (from 2mm up) Can be used on cords for hats etc as well as mask elastics. Sold in a pair.