We used a combination of leftover scraps (from the deck), reclaimed boards (shipping pallet wood) and new wood (three sheets of plywood) for the project. Including hardware (hinges and handles), the box cost us about $100. We stained the wood ahead of time and then built the box in a day. In it, we have room for plenty of wood, matches, fire starters, a grill and, of course, the fire pit.
I've mentioned before that having a designated place for all our things is part of what makes tiny house living comfortable. That applies both inside and outside the house.