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 Photos are shown below.  The part was made from a foam master which was then covered with fibreglass, slit in half horizontally, foam chewed out, cleaned up, and then bonded together with the flapper valve captured inside.  The flapper valve shaft is made from 1/4 inch hydraulic tubing.  It took a lot of fussing to get the flapper valve to seal without leakage (no obvious daylight when the flapper is seated).

 

Fred Moreno