Standard vent bung, in case the reusable one doesn't work or work well.
I used this to make the Picnic Cooler Mash tun found on the AHA website. I've used this over a year now and it still works, seal is great on the cooler.
I used this bung to convert a 48qt cooler into a mash tun. Remove the drain plug, glue this in place (with food safe sealent of course) and run your tube theough it. Works great and is much cheaper than other options.
If you are carbonating/conditioning in mini kegs this rubber bung is a must have item. I use this to seal the keg after I add corn sugar to carbonate. I use a small amount of corn sugar to simulate cask conditions.
I bought this when I built my MLT and it fit perfectly and the 3/8" High Temp Tubing (H8110) fit perfectly inside. No leaks and I didn't even apply any sealant.