This is a bag extender. It's basicallly a bag that straps around a regular backpack so you can have more storage. In my case I wanted to keep my cycling gear in a bag separate from my school stuff so it won't get wet. The bag has an insulated inside to keep the wet in and it's made of recycled materials. Best of all it's orange!!!
Step 1: Sizing!
So first off what kind of things do you want to bring along and how big are they? I wanted to carry my cycling gear which includes my helmet, running pants and jacket, underarmor shirt and shoe covers. I'm biking in the winter so it will be all wet and slushy so I wanted my bag extender to have some kind of water resistant barrier.
I took the largest object, my helmet and a piece of fabric and wrapped it around my fully loaded school bag, math textbooks and all. This should give you a general size as to how big the external bag should be.
My bag is made in 3 parts, outside bag, inside bag and water barrier. All three of these layers are sewn together to form one nice solid bag. There is an elastic drawstrap at the top that closes it all up. The extender bag is held on to the school bag using two straps with clips on them.
Step 2: Making water resistant
To make the bag water resistant I used a layer of plastic sewn to a fabric layer. The plastic layer is made of old plastic shopping bags melted down to make a tyvek like material. I got the idea for the material from a Make weekend project: http://blog.makezine.com/archive/2007/06/make_a_messenger_bag_out_1.html
The fabric/plastic layer didn't look so good by it self so I put this layer into another fabric layer.