I had a plastic cap on my power steering reservoir that leaked a bit. It didn't seem to have any sort of rubber seal on it. I ended up grabbing a plastic cap from a mid-90s Chevy truck at the wrecking yard. It looks identical to the one I had, but it has a rubber flange/diaphragm part-way down on the dipstick. This cap does not leak. You can still buy this cap style new from GM (part number 26095243), although they've switched to the goofy hieroglyphic markings that GM started using on their vehicles circa 2003. I bought one of these from the local Chevy dealership for my '89 Chevy. I did test-fit it on my '68, and it fit fine.
https://www.summitracing.com/parts/ado-26095243