Option 1: Wrap fish in a damp towel and place inside a large plastic bag. Seal the bag and freeze for 2 days. When frozen, remove the fish from the bag and trim away any excess flesh. Taxidermist can then mount the fish as they see fit.
Option 2: Freeze whole fish in a single layer on a baking sheet. Return to freezer for 2 days. Once frozen, remove from freezer and cut into desired pieces. Taxidermist can then mount the fish as they see fit.