In the original cupola design for the Tiger, the top flat ring was held in place by 10 bolts, equally spaced around its edge.
This German-drawn cross section shows one of the bolts. It goes into a threaded hole in the main armour ring of the cupola. There is no possibility of using a nut here; the hole is not accessible from underneath.
This is one bolt in a real cupola. As usual, there is a locking disc under the bolt head, bent into shape to hold it in place.
This is how the same item is represented in popular model kits; Dragon above, and Rye Field below. It's clear that both manufacturers have represented a nut instead of a bolt; there is a short length of bolt tail protruding through.
You can increase the accuracy of your model by sanding down these items to represent flat bolt heads.