I have felt that the best way to euthanize a large fish is a baton-blow to the head. That is too hard for most people.
The other two methods include placing the fish in a freezer In a suitable size container that prevents the fish from being terrified. Hypothermia sets in slowly and the fish perishes painlessly. (They have no spinothalamic tract and they are unconscious by the time freezing occurs.)
I think that would be a very large container to fit into your freezer.
The third method involves oil of cloves, that is available anywhere they sell essential oil‘s such as General Nutrition Co GNC. You simply put a few drops of water per gallon into a ziploc or jar, shake the container very well (I’m thinking Ziploc bag) then pour the emulsified oil of cloves and water into a container with the fish in it. The oil of cloves anesthetizes the fish at which point you can slide it into the freezer without the water. No fear.