I went out with a guide about 6 months ago and caught a bunch of crappie. He recommended that we use effectively this approach but with plastic bags instead of milk cartons, making sure the fillets were completely covered with water.
The resulting bags are irregularly shaped and take up a lot of space in the freezer, but the fish taste great when thawed and cooked. We ate a bunch the day after we caught them and then finished them off 6 months later. I couldn't taste any difference in flavor.
I don't have a vacuum sealer but I may end up getting one, in which case I'll try that approach.