Bigger on the inside, Arifureta Vol 5

Hajime ducked underwater and pulled a small, cylindrical fifteen centimeter long object out of his pocket. It had a snorkel and a mouthpiece attached to the middle. The canister was actually an oxygen tank. He’d used creation magic to enchant the canister with air magic. The space inside was much larger than it seemed, all filled with air.
He’d originally wanted to make the space as large as his Treasure Trove’s, but he hadn’t had time. While in Erisen he’d prioritized repairing his damaged equipment.