I didn't like the idea of a hanging padlock so drilled a hole in the side and put a simple turn lock in and the tongue goes into a slot cut into the seat base. The base should be vented anyway to allow air to escape. the lock goes into the narrow part and I put a small alloy strip inside to stop any small items getting in the way, this was just pop rivetted from the front so that it does not show. The tray cannot fall out but more than that stops idle hands having a look.