A loin cloth would do. There are no clothing rules. Most shooters wear long-billed hats. You'll get one very soon either from the club, a new friend, or manufacturer who sponsors trapshooting. Generally, just wear what you want.
Shorts are fine.
Bring sunglasses with you. Dress for the weather. You can make money in trapshooting and many shooters do. You'll need at least 3 to 6 months of practice on weekly Sunday's first. Hey, now you know why the rich people wanted to keep this sport all to themselves.