Position two 30-inch boards on edge, parallel, and 15 inches apart. Set two 15-inch boards perpendicularly between them on edge, so they are 27 inches apart and all corners are flush. Screw through the 30-inch boards, and into the 15-inch boards. Use eight screws for this task, assembling an 18-by-30-inch rectangular frame.
Position a 45-inch board perpendicularly inside each corner of the frame, with the frame lying flat. Stand the boards on their ends. Screw through the 15-inch boards, and into the 45-inch boards. Secure each 45-inch board with two screws.
Set the last two 15-inch boards inside the frame. Place the boards parallel with the other 15-inch boards, and against each pair of 45-inch boards. Set the 15-inch boards 17 inches apart, with their top edges flush with the top edge of the rectangular frame. Screw through the 30-inch boards and into the 15-inch boards using a total of eight screws.
Lay your plywood flat on the frame so all of the edges are flush. Screw the plywood to the boards on the frame. Use four screws for each side, and space them 4 inches apart.
Place your firewood in the stand so the pieces are perpendicular to the 30-inch boards. Do not stack the wood higher than the 45-inch boards.