It's cheap, it's easy, and you'll never cry if you wreck it. It's the Jack Bag!
Step 1: Tools & Supplies
Tools & Supplies:
Stuff not pictured because I'm disorganized:
Iron-on patch compound
Step 2: Make a Paper Stencil/guide
I ended up with the front and the back (the rectangular pieces) being 11" x 13". I also rounded the bottom of this piece for purely aesthetic purposes.
The long strip that connected the front to the back (and makes up the bottom and sides of the bag) was 3" x 36" (I cut it extra long; better too long than too short).
When I was satisfied with the dimensions, I cut the model apart so I could use them as a pattern on the cardboard.
Step 3: Cut Out Your Pieces
Step 4: Start Assembling
Starting at a seam, use duct tape to attach the thee pieces together. I used two ways to join the pieces at the curved sections.
At spots where the curves were, I used skinny pieces of tape so that it could follow the curve (I included that in one of the pictures).
Step 5: Make the Flap
Cut the paper guide down just a tad so it is smaller than the main body of the bag, trace it out onto cardboard, then cut it out.
Step 6: Attach the Front Flap
Pull a length of tape out and lay it on the floor, sticky side up. Lay the back of the bag on it. Stand the bag up and loop the tape over the front loosely - try not to let it stick to the top, but fix it to the front flap.
Step 7: Securing the Flap
Build up more layers of tape until you have a solid structure running from the front flap to the back side of the bag.
Step 8: Reinforce Everything
Step 9: Add a Closure
A bag closure is a good idea. I had some magnetic tape around, so I added it to the inside of the front flap, then added corresponding pieces to the front of the bag.
Step 10: Add the Strap
First I measured myself with the bag and pulled out a piece of tape for about 31". I layed it out, sticky side up, over the floor and stuck another piece of tape on it, sticky side down.
Step 11: Plan the Strap Attachments
I studied another messenger bag for a more ergonomic design to attach the straps. The picture shows it best - if you have your strap attachments come off at those two points, the will wrap around your body better.
Step 12: Make the Strap Attachments
After you have done this, you can trim the excess off.
Step 13: Attach the Strap to the Bag
I usually have bits and pieces from old bags laying about that I use for bag construction/repair, but this time I had clothes line. I thought about making it quicker and just taping a strap onto the bag, but I feel that the rope joint will absorb some of the wear and tear.
Using a hole punch, I made holes in the end of the strap and the strap attachments, threaded the line through, and tied a double overhand knot (though you can use what you like).
Step 14: Extra Stuff I Didn't Do
I like the raw cardboard/duct tape look that I came up with. However, you can have a more polished look to your bag. In the fabric section, they have iron-on patch compound. It comes on a roll, with a plasticy material on a kind of wax paper. The compound melts under an iron and holds materials together (like hot glue, but in sheet form).
Using the manufacturer's instructions, iron the material onto your fabric, allow to cool, then peel the paper off. Place the fabric on the cardboard, patch material touching the cardboard/duct tape. Situate the fabric how you want it before you iron it on - you may want to fold the edges of the fabric underneath so they don't show. If you fold it, you will need some more compound on the underside of the fold so it will stick to the bag as well.
If that sound too convoluted to follow, I will make another guide to doing that. Good luck!