After extolling the virtues of Lee Valley and its Finnish birch plywood, the last batch proved to be not suitable for the tray bases. This plywood was warped. The first tray base I worked on was 9" by 12" and was reasonably flat. The photograph shows the finished tray. The next tray I finished was 12" by 16" and did not turn out well at all. The base was too warped. It was so badly warped I could not put it among my sale items.
I took the plywood sheets and the remaining warped bases back to Lee Valley. They accepted the returns and refunded my money. The only sheets they had in stock were also badly warped.
I have since visited another supplier of plywood, "The Wood Source" near Manotick, some 30 kms from home. There they recommended 1/4" plywood as providing a more solid base with little chance of warping. I will try again.