Holes and slots designed to relieve water pressure. Additional holes would be incorporated to bolt onto tool.
Tight fitting diameter to administer aggressive plunging action while releasing flappers when excessive pressure is realized.
Less tight fitting for compromised wells, wells with evidence of age, corrosion and deterioration.
Ideally 1/2 to 3/4 in. thick rubber, two disks at 6 to 12 inches apart on the top of the tool and two similar disks placed at the bottom of the tool.