One roll of 1" wide micropore tape.
Micropore tape is the easiest way to keep bugs and competing organisms out of your grain spawn jars while still allowing for gas exchange during the colonization process. Simply drill a 1/4" (6mm) hole in the lid of your spawn jar and cover with a single layer of micropore tape.
Price is per single roll of 1" (2.5 cm) wide micropore tape. Each roll is 10 yards (9.1m) long.