Billionaire Elon Musk claims to have made $300,000 for The Boring Co. — exclusively with hat sales promoting his tunneling venture.

Musk, the chief executive of Tesla Inc. TSLA, +1.63% and founder of Mars-focused SpaceX, founded the tunneling company out of his frustration with sitting in Los Angeles traffic. The idea is to run a Hyperloop that will shuttle cars in underground pods at up to 150 mph.