I was making gelatine blocks with Knox gelatine, using the recipe for Knox Blocks on the back of the box, but substituting 1 cups sugar and 2 cups water for the juice. After it set up overnight in the frdge in a shallow pan I cut it into 1" squares. Add enough cubes to get about 3" deep in the bottle, or maybe halfway, no more. Let that get back to room temp and add lukewarm water to a few inches from the top. Use wine or beer yeast, 1/4 teaspoon is enough. Keep the extra cubes in the fridge, sealed well, they will keep 4 weeks or so without molding. Should last about 3 to 4 weeks, sometimes more.
Do watch it at first to be certain it does not foam.