This thing was made with Tinkercad.

(Where the hell's the picture?)
We use these to protect the threads of SCBA (firefighting air packs) bottles during storage and transport. They are a cheap little plastic cap that costs nothing. Would be fun to put someone's head or a skull on the cap. The plain white tapered cylinder is the internal dimensions of a Scott SCBA bottle cap - use it as a "hole" to make anything into an SCBA bottle cap. Different manufacturers may have different dimensions.