There are many ways to steam fish. I do it with banana leaves, idiyappam trays and a pressure cooker.
A fillet of Seer fish. To fit it, I cut into two halves.
Made a paste of cumin, coriander, garlic, green chilies and salt.
Rubbed it on the fish and wrapped it in the banana leaf. If you warm the leaf on an open flame for a few seconds it becomes soft and much easier to shape.
Pressure cooker with gasket but no weight on it. Ten mins. Let it cool a little before removing from the leaf.