Adding to my previous answer, the folded (rolled) paper towel, goes inside the bag, above the fillets, below the seal. This eliminates the prefreeze step for me.
Just pat them a little bit, leave a little moist, and put in the bag.
I double seal both ends.
Freezing in plastic ice cream gallon containers, with a little salt, works fine too.
Just depends how my back feels, and how late I fished.