I have had snake egg shells that I wanted to preserve in the full oval shape after they hatched, and what I did was thoroughly rinse the slimy contents, stuff some tissue paper inside to preserve the integrity of the shape, then after they dried some, sprayed the outside of the leathery shell with polyurethane. This did a good job of preserving the shape, and not allow it to shrivel up as it normally would. But if the contents are hard, as some infertile eggs are, I don't really know what to tell you. It "could" still work for them too, I don't know.
Otherwise, they might be best preserved by more standard methods in a jar.
~Doug
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"Better to be silent and thought a fool, than to open mouth and remove any doubt!"