Power steering pump gasket for cap leaking. The old gasket is all dried up

I have the cap. There must be a way to make a simple gasket for it. How do I know if I buy a used one on eBay that gasket is any better than the one on mine? Aren't the used ones 40 + years old maybe?
Body Person thanks. I have a piece of heavy duty rubber. An industrial type of rubber that should hold up to power steering fluid. I'm going to try that and see what happens. I'm hoping the thickness is ok. We'll see .