It is a bell shaped piece of rubber with a groove for a stainless steel clamp around the lower portion to stabilize it on the casing. 

The outer edge of the bell seals against the drill hole forming a collection point for the material used to pack off the top of the
well casing. We offer stainless steel clamps for use on our shale trap/ formation packer to hold it on the casing. We use only an all
stainless steel clamp with an all stainless steel screw to insure that rust will not allow the clamp to weaken or break causing the
formation to fall.