"An eye cap is a simple ten-cent piece of plastic. It is slightly larger than a contact lens, less flexible and considerably less comfortable. The plastic is repeatedly lanced through, so that small sharp spurs stick up from its surface. The spurs work on the same principle as those steel spikes that threaten Severe Tire Damage on behalf of rental car companies: The eyelid will come down over an eye cap, but, once closed, will not easily open back up. Eye caps were invented by a mortician to help dead people keep their eyes shut."
--Mary Roach, Stiff
BTW: a very informative, and funny, if somewhat macabre work!
My mom was a nurse and she trained in the 1940s. She said the nuns who taught her and her classmates also taught them how to prepare the dead, and one of the things they did was to put a few fibers from a cotton ball under the eyelids to keep them closed.
Maybe that doesn't work as well as eye caps, I don't know. But it does seem a bit weird that someone went to all the trouble to invent and mass-produce eye caps.
I don't want nobody stealing the pennies from my eyes when I go. Everyone knows that they are left there as the fare for Charon's ferry across the Styx. Schwarzfahren in der Nachwelt? Hardly an auspicious beginning.