Engine coolant recovery tank cap

I have a 1987 Lincoln Town Car. The "CAP" on the coolant recovery tank is a rubber PLUG. The hole opening in the tank is about 15/16" or 1" in diameter. I checked online in a parts book and they show the tank but not the cap or PLUG. Strange Ford would use a rubber plug on this tank and not a CAP that twists or screws on. Any ideas where I can find one? Junk yards don't seem to have any cars around this year. I'm restoring this car and would like to find one.
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