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: 
This may seem to be a simple approach but here it goes. 2 options:

1. 2 separate coolers, one for beers one for food. If you don't open the food cooler till the morning the cooling will last much longer.
2. The easier option: place your beers in the freezer as well not long enough for them to freeze but maybe 5 hours they would drop to -4 > -7 degrees, but not freeze. Therefore bottles or cans are not damaged but cold enough that they will double the life of your cooler as they will continue to cool all the time they are in there.
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.