Does my sandbox need a bottom if it will be over clay soil?
I made a sandbox a couple of decades ago for kids. It had a plywood bottom and was built with legs that held it up about seven inches off the ground. The bottom had drainage holes to let water out. The plywood lasted for a long time. How long? Hard to say I sold the thing in a garage sale after 10 years when the kids grew out of sand boxes.

BTW - I had a lid over the top made out of two pieces of plywood that were hinged together. The lid kept out cats and most of the rain. If the sand got too wet just opened it to the sun and worked the sand around with a small garden rake over the course of a day or so.