Well nuts are a rubber body threaded inserts / rivet nut. 

Well nuts are a rubber bush with a concealed brass nut at one end . They have a flange on one side with a barrel that is inserted into a hole. Well nuts are an ideal option for weatherproofing holes or isolating vibration. . Check out nutsert blog. 

Well nuts are a rubber body insert that consists of a neoprene bushing with an enlarged flange head on one end and a captive brass nut embedded on the inside at the other end. An excellent seal option against air, moisture, and vibration, which makes it ideal  in lightweight bearing applications associated with fibreglass, plastics, or composite sheeting. Also referred to as a rawl nut, rawlnut, or a rubber nuts  Check out the nutsert blog. 

Great when seeking a seal against air, moisture and vibration.  

Well Nut Lab - How To Install.

Easy to install, no special tools required, just follow these simple steps: 

1. A hole must be drilled into the installation material. The hole will need to be the diameter of the rubber portion of the insert.
2. Slide the insert into the hole so that the flange is the only portion of the nut exposed. At this point, a hole must be drilled into the material you wish to attach if it has not been drilled already.
3. Place a washer onto the machine screw. You want to use a washer to press down on the flange portion of the well nut. Otherwise, the well nut will try to spin during installation.
4. Line up the two holes and insert a machine screw that matches the threading of the well nut.
5. Begin driving the machine screw (and washer) into the threads. As the well nut tightens, the rubber portion will start to deform and pull against the back of the installation surface creating a tight strong hold.
6. Congratulate yourself. Job well done! :)