lol T-shirts are a very easy go to, cheap to make and give you a ton of options to customise etc. Classic Utopia t-shirt would probably be bought by a lot of people just for sheer linkage to the game. Old classic top kingdom names might be another option. I think largest success would be play on words/actions from the game.

Examples:

Watch my Plunder
Abduct that science
Razing your acres
Fireball your pessies
Raze to the ground

etc etc etc