Get some 1/4" plywood (or whatever material you like) - I used a sanded birch 2x2' sheet from Menards, cut it down the middle, then stained each half different shades, varnished, and then cut.
I provided a file with the fish int he frame, because I know someone will ask for it. But I prefer to cut the fish separately, using Martin's layout, which preserves the wood grain for individual fish and gives a nice visual texture to the finished piece. If you are not using wood, then I suppose there's no reason to prefer it.
Optionally, cut the frame parts, glue and clamp, and fill with fish!