Tonneau -- Bed Cover Wanted for 71 Chevy C20

I've been searching on line for a top-mount(rail system), snap-on, tonneau cover for my 1971 Chevy C20 long-bed, but haven't been able to
find one. The only ones I've found use velcro (hook & loop) that has to be glued directly to the box. I want snaps. Similar to this:
Can anyone point me to a web page that sells these things for older trucks? I can't find one made for anything earlier than 1973.
Thanks, Jerry
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