Here are my designs from the drawing board:
You can see that this is more than just an ordinary bike with a sidecar added. Due to the size of the cooler that's needed, it necessitated an extra-long bicycle to fit alongside. So my first phase of the project was to build a longtail bike.
Then I went on to build the box frame out of recycled bike tubing, which has used up at least 8 different bike frames so far. Once I had the big rectangular box together, it was attached to the longtail and set up for some test rides.