This article is published without any guarantee. We have made ours since about 1962, the ones I had first were a lot older than that. It's also about the maximum for reaching down to shake hands with children.
Hardwoods are much too heavy.
Bolts, wood-screws, washers, tacks. Mark a narrow face at 20cm from one end and the opposite face at 30cm from that end. Use a pencil to connect the marks across the wider faces, and saw across. More exactly, level with the bone that you can feel about an inch below the centre of your kneecap. Mark, or get someone to mark, the level where your instep comes.