I don't use a fish bag, and I'm not sure how cheap and oakie you want to go (which would be my route). But I've read online about people making their own with
reflective foil bubble insulation and/or using a water tank insulation jacket made of the foil bubblewrap.
One states:
"A few suggestions...
Pick up some silicone adhesive for the seams that you plan to seal. It's flexible, waterproof, oil proof, etc. After running a bead of the silicone put a 2" x 4" on the seams with some weight on top. After the adhesive cures, use your foil tape to cover the seams.
When you put a fish in the bag, you'll want to cinch it closed to keep the cold in. Tape doesn't work well on a wet bag, and you don't want to tie it with rope/docking line as that will bruise the side of the fish that's on the deck.
I have no idea how well these suggestions would work. If you have a heavy duty sewing machine, it would be cool to make your own by maybe getting a bunch of those cheap, big foil lined lunch bags and cutting them apart and sewing them all together and attaching a zipper and straps? I've made bags and totes on my old machines (got rid of them when I moved
) and they are pretty easy to make, so I don't think a fish bag would be that difficult.