The concept is a play on the excitement surrounding prediction markets in the blockchain community, so feel free to draw inspiration from those topics.
Create a "official seal" type design for "PROOF OF SHIRTS" that will occupy the center of a standard print shirt.
The seal should have enough detail to be appealing from a distance but reveal additional details on closer examination
Additional text you can use: "The Wearable Prediction Market" "Bots Don't Wear Clothes", "http://www.proofofshirts.com" "In Crypto We Trust", "Vires in Numeris" "100% Guaranteed".
Ultimately the goal is to make an interesting, funny shirt for people in the cryptocurrency community.