What an interesting assignment, and I'd like to commend you for doing your own work and only using us to help you when you got stuck. I appreciate your diligence. These are, of course, going to be the more difficult letters to find in Lord of the Flies, but I'll do my best; Here goes:
Q - Simon is the one boy who tends to get queasy at odd times throughout the novel. He faints when we first meet him--something he apparently does with some regularity--and he faints again in the incident with the Lord of the Flies.
X - Piggy is the only one on the island who really wants to eXamine each of the boys when they first arrive on the island. (Weak, I know, but it's better than nothing.) The older boys eXtinguish the sand castles, Jack eXamines pig droppings very carefully....
Y - The boys all yell like savages (the book refer to it as ululating) at the end of the novel. (Does Jack paint his face with any yellow? Can't remember.)
Hope this gets you thinking, anyway!