A very cheap thin-walled 18mm socket, the bulkier better made sockets will not fit down the hole.
Strength isn't the problem, the problem is getting the socket to physically fit down the hole where the spark plug is.Actually you pay more for a thin walled socket. The cheap ones need to be thicker to be stronger.
Its only the upper edge of the socket that is really the problem. IF you grind the upper edge round it will be a great tool for the job. No matter the wall thickness of the socket.