The simplest noon mark that I have is a short strip of white plastic strip which is attached to a brick wall. At Local Apparent Noon, the shadow of the side of my house crosses the strip, as shown
A much better noon mark is my analemmatic noon mark, which shows the time of “clock noon” (or 1pm when British Summer Time is in operation). The gold plated sunburst was made for me by Tony Moss of Lindisfarne Sundials, and has a 2mm hole in the centre, the spot of light indicating the sun’s position.
I made this dial empirically - rushing out at noon and knocking in a small brass pin each day that the sun was shining. It took about 2 years to get the full analemma completed. The vertical line indicates Local Apparent noon, and the (almost) horizontal line marks the path of the spot of light at the equinox - it is not horizontal because the wall declines 14 degrees east of south. You can see that sundial time is the same as clock time on only 2 days each year at my longitude. The picture below is a close up of the spot at noon on 6th October, one of those 2 days. It will indicate noon to an accuracy of about 15 seconds.