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.