Jackcap 6.3mm 1/4in Silicone Rubber Dust Plug, 
Sold loose, price is for a single plug.
Jackcap is a soft rubber plug that simply pushes into the empty slots of your musical instrument keeping it clean of dirt and debris making sure that when it is finally time to use that channel it will be as clean as it was when it was new. 
They are not hard or brittle, they are soft silicone rubber and very pliable, they will absorb knocks with ease and they will not fall out even if turned upside down,