I have used this pattern for shopping bags and it is easy to scale up if needs be
http://www.madeeveryday.com/2012/07/...ked-tote.html/
The one I use for vegetable shopping at the local market I ice dyed an old piece of chair canvas and interlined it with an old waterproof lining. The last thing you want is anything that leaks spoiling the outer fabric. I would just make sure any fabrics you use can be washed. The bags will get dirty especially in the boot of a car.
Hettyb