I think I may have something.

let me run it by you.

The three stars of orions belt to the naked eye are only inches apart - it's possible using the old pen and thumb to draw them to reasonable accuracy.

But how to you scale this up to a mile long monument complex?

If you drew the stars on the ground, then atached some straight rods to them the easiest way to scale this up is to cross the rods over with a movable hoop in the middle. That way as you move the hoop closer to the drawn stars the greater will be the distance between the ends of the rods but they will still retain the scale.

Once these rods have been set up correctly, simply extend them in order to give one of the alignments needed for the monuments.

This does not explain how the worked out the length of each rod extension but it seems to me to be the sort of trigonometry that could be performed with rudimentory tools and also explains why we have a mirror image of the stars.