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.
The "R" value of the cooler insulation as well as the seal around the lid are important in retaining the cold. The size of the cooler is also important because the ratio of ice to the product you are trying to keep cool is also a factor. You can get pink RV anti-freeze used to winterize drinking water pipes and tanks in boats and motor homes It is I would assume non-toxic if accidentally ingested. Ice packs made from anti-freeze would probably cool faster to a lower temperature but not for a longer time.