I don't understand what you would need that for. If it's too hot to be outside, it's an hour before noon or up to three hours after.
Measuring time has always fascinated people and while quite a few conduits have been used to tell time, the sundial has been considered more reliable since it it not always clear what the conduits were meant to do and might change unexpectedly.
The construction is very simple, but there are not many who know how to position them correctly in order to make sure that time is correct.
According to a manufacturer of ornate sundials, the trick is to align it in a certain direction using the stars as a guide. After aligning it and making sure that it is affixed the goal is to paint the timelines, i.e. the lines representing the hours. This is easiest to do by drawing small marks when an hourglass runs out as a reference.
In many towns there is a sundial in a public place, sometimes it is a part of a piece of art, like a statue that doubles as a sundial. These sundials are often also good meeting places because everyone knows where they are and it is obvious that something is a sundial when one sees it.
The obvious disadvantage is that they only work on sunny days, which is why sundials are more often employed in the desert. At night one could also estimate time by observing the stars, but again, it needs to be cloudless.
While manufacturing isn't difficult, just put a stick into a solid plate at an angle, the difficulty lies in knowing the direction the stick should point, as well as knowing where to draw the timelines.