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!
The book of Psalms is a great book. Many of the Psalms have to do with God bringing victory to his people in the face of their enemies. If you research some of the Psalms that King David wrote you will find good examples of this. King David wrote a lot about how things were going good for his enemies. At times it seems like David was even a little depressed, infact he probabaly was discouraged. Usually the ending with reveal that David encouraged himself in the Lord and continued the fight.
There are also many Psalms about praise and worship to God. You can find line after line that describes who God is and what He has done. Many Psalms are actually put to music and become songs of praise.
Another major topic in the Psalms is the provision of God. You will find that much is written about God as a provider. When God intervenes in the situations of man and does the impossible, Psalms are usually written. There are also some Psalms that are about man asking God for help. Many times as you read Psalms the writer will ask a question in the beginning and by the end they have the answer.