I have been using a prototype as often as practical for school and it has performed admirably--when not failing. It took a few weeks to recognize and repair most of the weaknesses and has operated stress free for most of the semester. It's also survived a couple of semi-rigorous bike rides.
This tutorial is based on the bag I've been using, but with important changes that will improve durability. Also, this is the most basic design I've made. Version 2 can be easily customized to suit the needs of the builder.
Here are pictures of the model currently in use (v2.2) and the one made for this instructable. Version 2.2 has taken some abuse and has underwent many, many repairs.
Don't let the 19 steps daunt you--making a messenger bag from scratch can become complex. I've attempted to simplify it, despite the many steps.
Step 1: Gather Materials
1. Duct tape
---3 30yard rolls.
3. Cutting board.
5. Yard Stick.
I imagine any brand or color of duct tape will do. For this instructable and my previous bags, I used standard Scotch/3M brand tape. It is 1.88in wide, gray, and purchased at the local hardware store. For your first go at this, I suggest whatever is least expensive.
The scissors, cutting board and pins can be purchased from any sewing supply store. If you don't want to pay for a cutting board, it can be substituted for a flat sheet of corrugated card board that has linear dimensions more than 3ftx3ft. I haven't tried it this way, but it should work. Also the pins can be substituted for thumb tacks--you'll need at the very least 44.
If you sew, you probably have good scissors, a cutting board and pins readily available.
Please note: don't use your good scissors and pins to do this instructable*. They will become gunky from the duct tape's sticky substance and annoying to use later on. I suggest after using pins for the first run that you separate them from your other sewing pins. Use a less good pair of scissors if you can.
After cutting a few pieces of duct tape, your scissors will gunk up and cutting performance/quality will diminish greatly. It is worth the time to take a moment to clean the gunk off of the scissors so they can cut cleanly again.
The materials needed to construct the hardware:
1. Duct tape
2. Marker Flags
3. Tension Pins
6. Tin Snips
7. Sacrificial Ruler
8. Regular Ruler
This portion is optional! If you have hardware laying around, you can easily use those.. I don't recommend it though.
All materials can be bought at Home Depot, except for the ruler maybe.
Hardware includes a handle anchors, shoulder strap anchors, and buckles. As mentioned, you can salvage these from another, preferably useless, bag. I strongly suggest that you don't ruin a perfectly useful bag just to make a mostly useful bag. That would be silly.
Amounts of different materials will be discussed in the hardware portion (step 12).
After gathering materials to build the bag structure, you may move to the next step without fear of injury.
*As user Aggrieved points out, you can periodically clean the scissors or pins with eucalyptus oil or goo remover. Be careful when cleaning scissors with plastic parts, as goo could dissolve these parts.