I might be a tad late with this, but i use a very similar method that Martin has described.
I just use a pole on the surface making sure it is spot on square and level.
I get the Solar noon from here and found it to be a good site, you just put in your location,
Also shows you what planets are available to see and a few more things.
https://www.timeanddate.com/astronomy/@11746044
https://www.timeanddate.com/astronomy/night/@11746044
Might be of some help.
Cheers.
Peter.