Place flowers and citrus slices in the water, wedging them between the bottle and the inner wall of the milk carton to keep them submerged. It should be a tight squeeze, and you may need to use the back of a utensil to shove the flowers and fruit to the bottom of the carton. You won’t be able to see your arrangement through the carton, so mentally take note of what fruit slices and flowers you’ve already used so you have an even spread and variety of colors.
Once flowers and fruit slices are tightly packed on the bottom half of the carton, fill with more water until it is mostly full, and repeat the process with more fruit and flowers. The water will rise slightly, so do not fill water to the very top of the carton.
Place in a flat, level area of the freezer so that the carton can stand upright on its own, and freeze overnight.
When you are ready to display the block, remove the milk carton by running it under cold water briefly. Pull the bottle out of the carton by the neck. If desired, chip or scrape small amounts of ice from the corners to give the block a rounded appearance.