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.
Step 3: Finishing the Bag
Now that all the layers are all sewn together, the bag shape can be finished off. Sew everything inside out and twice so that the bag wont come apart and the edges are all nice and wont fray. Also while your here come up with the method of closing the bag. I used a elastic drawstring with a springy clip thing to hold it all together. The string is held in place using a folded over top edge of the bag. When that's done turn it right side out.
Step 4: Adding Straps
Now that the bag portion is done, we need to add straps to attach it to the main bag. The straps will reach around the main bag and attach using plastic clips.
I placed the extended bag on the outside of the main bag and marked the places where the straps would be sewn to using a pin. I used some hardcore zig zag stitches to make sure they wouldn't come off. The straps are pieces of scrap fabric that I folded over and stitched up.
Step 5: Finished!
Strap the bag to the other bag and load it up. I got all of my gear in the bag with some room to spare. The straps are adjustable to accommodate for any differences in the main bag's size. I used some safety pins to make sure that the straps don't loose any tension and make the extended bag slip. I have yet to test it out but I will get my verdict up as soon as possible.