I think the mistake many people may be making on the normal "freeze the fish" method, may be that they are simply placing the fish in the freezer in a bag. I could see that causing a lot of stress by putting the fish in direct contact with ice, as well as not being in water to breathe.
as the link i posted above suggests, even after a dose of clove oil, the fish could possibly recover and so it's suggested that after they have succomed to the clove oil, to place that bowl of fish water with clove oil in the freezer with the fish still in. This would, it seems to me, ensure that the fish stays anesthetized until the water in the bowl and the fish freeze solid, ensuring the fish has been killed.
also, they say you can find clove oil at home brewery supply stores, or in aroma therapy stores. But I would be sure you are getting PURE clove oil, and not "Clove scented" oils.