This sounds like a great project. To make it sound like an authentic psalm, you should explore the book of Psalms and see what types of psalms there.
Here are some examples:
Psalms of praise. Psalm 100:1-3 "Shout for joy to the Lord, all the earth. Serve the Lord with gladness; come before him with joyful songs. Know that the Lord is God."
Psalms of lament. Psalm 22:14-15 "I am poured out like water, and all my bones are out of joint. My heart has turned to wax it has melted away with me."
Psalm of thanksgiving. Psalm 18:1 "I love you, O Lord, my strength. The Lord is my rock, my fortress and my deliverer; my God is my rock in whom I take refuge."
In light of this variation, you could be pretty creative. Perhaps, the most creative thing you can do is to combine the various types of psalms. Good luck!