I keep a coleman 36-quart cooler strapped down against the wheel well with husky hang-alls so it doesn't budge. It can hold about 24 gatorade/water/soda bottles. Make sure you keep the lid held down with a lashing strap. Ask me how I figured that one out.
I spend about 75% of my workday outside, and hated having to go to the convenience store to get drinks while on a job site. The only problem is that you can't open the cooler when the pull-shade-thing is in the back.
I keep about 3"-4" of water in the bottom to minimize heat transfer and add ice every sunday night. I also learned to freeze gatorade and water bottles, and put a few frozen ones in every few days. That way, I just keep cycling frozen ones in as I drink the thawed ones.
Saving up for a Pelican or yeti 45-qt..... one day