Think about the things that Ralph valued most in Simon. Make a list of these things, and then work to arrange these items so that it flows well. You've got some good examples in post #1.
As a brief statement commemorizing or epitomizing a deceased person, Simon's epitaph should include praise and remembrance of his salient characteristics. For Ralph, then, the most salient quality of Simon is his loyalty since in the final passages of "Lord of the Flies" by William Golding, Ralph "wept for the end of innocence...." This innocence is truly in the character of Simon, who because of it could not give words to his knowledge of the "beast" as the evil in the hearts of the boys such as Jack, Roger, and the other "savages."
Along with mentioning his innocence and loyalty as Simon always helped with the building of shelters and the fire, Ralph can mention Simon's inner strength. For, despite his sensitivity and fears, Simon realizes that the only way to deal with fear is to confront it. When no one else desires to climb up the mountain after seeing the beast in the air, Simon reasons, "What else is there to do?" Confronted by the beast, and imagining that it tells him "everything is a bad business," Simon, with fortitude, replies, "I know that." Thus, compared to Ralph's perspective of how things are, Simon is much more intuitive and insightful.
And, because of his clear sightedness, Simon becomes a threat to the savages, who kill him. A sacrificial victim to the evil in the hearts of man, Simon is the most loyal, the most kind, the most intuitive, the most stalwart, and the most human of all the boys on the island. His death, along with that of the rational Piggy, signifies the death of innocence and goodness.
Now, when you write the epitaph, you may wish to consider the characteristics mentioned above. Also, as your question states "Ralph's mourning," it seems germane to include the idea of Simon's death as that of the sacrificial victim/the death of innocence.
The first thing that you have to do is look at the relationship between Simon and Ralph. Then, you need to identify the characteristics that were good in Simon.
Simon was kind to the little children.
Simon looked for answers in a spiritual manner.
Simon was brave. He went to face the beast.
Simon was helpful.
An epitaph should be a positive remembrance of someone. Since it is on the gravestone it would not be excessively long, but it should let someone know something the person stood for or that was good about the person during his life.
I saw one the other day that said, "Here lays Pete. He never met defeat. Until now." It was unique to me that is why I remembered it. You want something a bit more relative to Simon as a person.