HaLeiVi, I think you are making two separate mistakes here.
Firstly, it is a common misconception that sundials indicate shaos zemanios. Actually, most sundials indicate regular hours since they are based on the hour angle of the sun around a north-south axis. While sundials can also be made to indicate shaos zemanios (at least when calculated from netz to shekia), this is complicated to achieve accurately throughout the year, and was not common. Wikipedia’s article on sundials is a bit short on information about this particular point, but there are other websites devoted to sundials where more info can be found.
Secondly, R’ Moshe’s shittah about different size hours before and after midday is not based on any gradual astronomical phenomenon that could be conceivably indicated on any kind of sundial. It is based on the fact that the day starts at alos hashachar which is far longer before sunrise than halachic nightfall is after sunset. Thus the day is lopsided, and halfway through it is before midday (defined as when the sun is at the highest point of it’s path). Rather than redefine halachic midday as being before astronomic midday, R’ Moshe redefines the hours, making a morning sha’a zemanis a sixth of the time between alos hashachar and midday, and an afternoon sha’a zemanis a sixth of the time between midday and nightfall.
A sundial calibrated to show shaos zemanios from netz to shekiah will in fact have a minor gradual change in hour length through the day due to the fact that the year is advancing through the seasons at the same time, but this change is almost negligible in the span of one day. R’ Moshe’s shittah stems from the fact that he calculates the hours from daybreak to nightfall as explained above.