My solution is a bar inserted into the bag and bolted on to the original thin bar that held the top rigid. The new bar supports the strap and takes the full weight of the bag. While it was designed with the express purpose of fixing my bag most messenger bags are similar enough that the solution can be adapted to another bag.
Lest I have discouraged you from the m51 bag let me say this: it is an excellent bag which has served me exceptionally. So far only the latch and strap have failed, everything else has been fine and has replaced my former bag fitting everything in despite being half the size, and I still haven't filled the map pouch or the all of the back pocket. It is comfortable, well sized, and filled with more pockets than I have ever seen. Add this to being cheep and it makes a bag that is absolutely worth having.
Step 1: Materials and Tools
Lexan sheet 12 x 24, .09 inch thickness. Weird thickness I know, but it was the thickest available at my local hardware store. The exact dimensions of the sheet are not super important. All you really need is a sheet wide enough to extend from one side of the bag to the other and have enough material to build up to a 1/4 inch thickness.
2 number 8, 1 1/2 inch eyebolts.
1 number 8, 1 1/2 inch bolt.
6 number 8 washers, I used finishing washers for looks.
Super glue which will work on lexan, if it works on fabric as well super bonus.
Total cost: about $25. The lexan sheet was the most expensive at $15, but I had a lot of leftover material, so it was worth it.
Power Drill, mine is old, I know. Its single speed with serious torque, but just about any drill will work. If you have drill press you have my envy and can certainly use it.
1/4 inch drill bit. The bolts I got were number 8, so they took a 1/4 bit. If you have different sized bolts you'll need the appropriate sized bit.
Screwdriver. This is really whatever type matches your bolts, mine was a number 2 philips.
Saw. This is flexible, any saw which can cut your lexan sheets. I used a miter saw, but almost any saw will work. Again if you have fancy shop tools like a band saw you can use them.
Measuring device. Whatever you got should work, I used a square with inch marks.
Clamps. Again whatever you got should work, I used the cheep clamps available at your local hardware store.
Step 2: Preparing the Bar
Before we insert the bar we have to make it. First things first, we need to measure. First check length. Mine extended about a fourth of an inch past either side, so I decided to leave it. If its greater than that it should probably be cut before you glue it together in the next step. Next check width. I measured the width of my bag top to be just over 2 inches, so I cut my strips to be 2 inches and have a little excess in the bag. You want excess or when you close the bag the bar will cut into the side and wear out the cloth in the hinge area. Measure and mark enough strips to stack them up into a 1/4 inch thickness. It took 3 for me. The simply clamp them to a table and cut away. When done the edges will be pretty rough. You can file or grind these down but total smoothness isn't necessary. i didn't have a file so I rubbed them against the brick and it smoothed them out pretty good.
Step 3: Inserting the Bar
Really quite simple. All we have to do is glue the sheets together, and glue that to the bag. My glue required the surface to be damp so I wiped on some water with a spare rag. Afterward the glue was applied sheet by sheet and clamped down to a spare piece of wood. The glue didn't hold completely, so it required two going's over. Do this as many times as necessary as the sheets have to function as one. When they all hold together, take them off what you clamped them to and apply glue (and it my case water) to one side of the bar. Place a piece of scrap wood long enough and thin enough (which is dependent on the size of your clamps) on the handle side of the bag. place the bar on the other and clamp it together. Be careful not to have the bar bend around the handle, loosen your clamps if it starts to bend.
Step 4: Bolting the Bar
With the bar made and glued it's time to add the real connection and the anchor points for the strap. First up is removing all the clamps and the scrap wood. Next remove any of the frayed straps. I left the one that was intact because I was still testing to see if my fix worked, which it does. Next up chuck up the drill bit and plug it in. You can drill from either the cloth side or the lexan side, it doesn't matter. select your side and start at one end about an inch in, or as close to the original flaps as possible. Drill through slowly, you don't want to melt the plastic. Once through put a washer and nut on your eyebolt tightening the nut as far as possible, and screw the bolt through the new hole, with the eye being perpendicular to the handle. On the opposite side of the bolt after it is through the other side put on a washer and then a nut. Hand tighten the nut down as far as possible. Take your screwdriver and insert it through the eye, leveraging it against the sides so you can hold it tight. Use then wrench to tighten down the nut until it's tight enough that it doesn't have any give when the eyebolt is tugged on. Repeat on the other side. Then drill under the handle for the normal bolt. Put a washer on the bolt and insert it through the new hole. Put a washer and nut on the other side. Slip the screwdriver into the top of the bolt, being careful to keep the handle out of the way. Hand tighten down the nut, then tighten with the wrench until flat. Congratulations. Your bag is now ready for use.
Step 5: Optional Step 1: Shortening the Bolts
As many of you have probably noticed the bolts are kind of long and risk scraping against stuff inside the bag and catching on stuff inside when trying to open the bag. For this you will need the handyman's favorite tool, the high speed rotary tool colloquially known as the dremel, after the company. A cutting disk, and a iron oxide grinding bit are necessary for this. First make an incision with the cutting bit all the way around the bolt a 1/4 inch above the nut. You want some thread left in case you want to take off the bolt and put it back on some day. No pictures of me cutting, as trying to take a picture with a moving dremel in my hand is a very bad idea. Afterward slowly cut off the bolt keeping level with the original incision. When all three are cut off switch over to the grinding bit. Grind off the rough edges from cutting and then grind a slant into the edge of each bolt and smooth them over. You want round smooth heads which won't scratch anything they touch.
Step 6: Optional Step 2: Take Off the Excess
There was still some excess length which can be a little annoying, and it will look much better if it wasn't there. Once again we are going to use the dremel and cut off the the excess using the cutting wheel instead of a saw, because there is greater precision with the dremel and the dremel fits into the small space without attacking the cloth of the flap as well. Wear a dust filter during cutting as cutting it like this produces a lot of dust. Slowly cut off the each bit of excess making sure not to cut into the cloth. Make multiple passes, don't try to cut it all in one pass or you'll break the disk. After you cut it it looks better, but it's still rough. Rather than leave it get out that grinding bit and smooth it out. Now to make it look nice and feel good to grab grind an angle into each edge of the bar and take off the corners. This way when you grab it it won't cut into your hand.
Step 7: Optional Step 3: Matching Colors
I use the phrase "matching colors", but I'm being fancy. I'm painting, plain and simple. The shiny eyebolts and semi translucent bar make for a bad, poorly made looking bag. Painting up everything to match the color of the bag will make it look a lot better. All you need is some paint to match the color, in my case black, some masking tape, a stir stick, and a brush. Place tape around the top washer of the eyebolt without covering the washer itself. Don't be sparring with the tape as paint on the cloth will look bad. Pop your can open, and stir it up as much as needed. Dip your brush lightly, and dab paint on making sure to get the washer, nut and insides of the eye. Wait until dry and then repeat on the other side, taping between the cloth and the edge I painted over the bolts to secure them. It's not as good as locktite, but the stress on the bolts is not so bad that I need real protection, the paint is good enough.