wood 18 to 25 mm thick (Southport PR8)

Minimum 4 foot long. To make new slats to fix an old garden seat/bench. Solid timber or exterior plywood would be fine. So the slats are 48 inches (122 cm) long x 2.3/8" inches ( 6 cm )wide x 18-25 mm thick. Any amount would help. I can cut the timber or plywood down from a bigger size myself. Many thanks.