Rubber Grommet To take an airlock

Small red rubber grommet.

Takes a standard airlock. You can drill a 13 mm hole in a cap and it will then take this grommet!

An easy way to convert those gallon water bottles from Tesco...

You can drill a hole using a pair of scissors if you're careful, take it slow and don't stab yourself. Use one of the blades as a drill and make the hole a nice tight fit for the grommet, too tight and the  airlock won't go in but you can always carve a bit more off! Just take it slow and try to shave the plastic off slowly!