Question: I'd like to make some extra icepacks to put in my coolbag. The 2 commercial icepacks it comes with don't maintain the temperature for long enough, so I've tried supplementing them with plastic bottles of water that I've frozen. This works reasonably well but are there other containers that would work better and is there anything I could add to the water?

Answer: 
I've never done this myself, but I've camped with people who have used dry ice to keep things cold/frozen over the course of four days.

Dry ice is much colder than regular ice, so you may actually have to insulate things you don't want to freeze from it. Keep it at the bottom of the bag, or maybe move it to the top if it isn't cooling as much.

It's a bad idea to hold dry ice with your hands, so think about plastic bags and tongs.

I would also say that dry ice is a "slower" coolant, so try not to open your cooler as much.
Place you whole cooler in extra isolation, like a good sleeping bag or several old blankets.

This works best if you do not have to open the cooler much or at all.  

You can take out the beers all at once and keep them cool by rolling them, individualy, in a big towel.  

That allows the cooler to stay at a steady temperature.