DIY Slurpees? (hack or a whack)

http://www.cleanandscentsible.com/2014/05/diy-slurpees.html
Attempt number one: The first time I tried this, it completely failed. I don't drink pop (New Year's Resolution) so I tried one with a Sparkling Ice and one with a Diet Coke (I did it for science). I came home school and quickly rushed to try to get this done. I took my drinks from the refrigerator and shook them for ten seconds each and placed them in the freezer.  In the instructions it says to place in freezer between 3 1/2 hours and 4 hours. Mine completely froze in an hour. I went back to the instructions and found that I was doing some of the steps incorrectly. When opening the bottle I didn't aggressively open it in one motion. Then close back up and place upside down. What I had was a huge mess and was really disappointed that it didn't work out. As summer is coming to an end and the hot weather no longer exists, I was hoping for a last chance of summer. The first attempt was a huge whack and just made a huge mess for me to clean up.
Attempt number two: This time I only used a Diet Coke and started from room temperature rather than from a fridge. For starters, this turned out a lot better than the first attempt, but it still wasn't perfect like the website I got the recipe from. Since I started from room temperature, it took longer to chill and I did end up freezing it again, but I figured it out. This time I also opened the pop in one motion, closed it and put it upside down. This time I also opened it over the sink, rather than the counter. It still ended up being somewhat slushie but not completely slush-like. I ended up keeping the pop in the freezer for three to four hours. having the magic number a little closer to four hours.
I think this a cute idea to keep the summer vibe around for a little longer, but we are going to have to face it sometime that its fall now. Rather than reaching for making these, I would much rather drive to a gas station or an ice cream store and buy them that way. That way it will be completely frozen and not mostly liquids with a little bit of ice that melts within two seconds of being out of the cold.