This tool was made to help snap on round inspection hole covers as used on almost all fabric cover aircraft, and some metal ones as the Maule in these photos.
The tool is made from a brad tool. This tool is used to squeeze brad into picture frames. They can be found in most wood worker supply catalogs. Remove both parts of the jaws of the tool. Save the screw that holds the top jaw part, because it is most likely metric. The big black part is made from a large PVC pipe fitting. I glued some rubber grommet material around the top surface of the PVC part so that it would not scratch the inspection cover. The PVC part is attached to the brad tool with a hardwood part and four #10 machine screws. The part that will contact the inside of the cover is made from 1/4" aluminum. The corners are rounder and it is held in place with the original machine screw. I put a small stick-on-felt dot on the screw head so it would not scratch the aircraft's fabric. I take a little effort to adjust the position of the PVC part on the brad tool to get the snap just right. Also I found that I had to put a slight hook in the aluminum part.