Basically, sand the faces of a new and pliant garden hose seal so it's a slip-fit into the De Persia. Sand the outer edges so it doesn't wrinkle when installed. That worked for me, but I already had a De Persia ball that fit. (There are two versions of the De Persia).
The plastic Sunfish ball can be made to work (they're available as a 5-ball kit) but the kit I'd bought produced a ball that was too small. I fixed that at the hardware store, and glued two "unrelated" rubbery washers together. At the time, I was still sailing, and had plugged the bailer hole entirely with a $5 "1¼-inch expansion plug" from NAPA. (Which remains the solution for me ).