Well it has been years since I froze any in water with the fins on since my wife prefers the boneless skinless fillets. When I did freeze as you mentioned it was in some sort of container other than plastic zip lock type bags. Country Crock butter tubs and the large Cool Whip bowls were used by my grandmother and of course re-purposed to freeze fish with fins. Not sure of your quantity or size that you are freezing but any plastic container that will fit your catch should work and be sure to leave headspace for the expansion of the water. Any containers you use will hopefully be able to be re-used. There are all kinds of Tupperware type containers out there now, and with Discount stores like Ollies and Big Lots in our area something plastic with a decent lid shouldn't be hard to find and hopefully will even be stackable.