With the other suggestion of the "load cell", is the suggestion to have this sensor on the top of the shoulder? assuming I am measuring the pressure produced from the contact of the strap to the shoulder?
however if the testing was set up to be in the same position every time,
Search for "fish scale" or "luggage scale" on ebay.Leo..
Are those bags usually carried in messenger bags? Lets be specific about what we're doing here. You're wasting our time if you're sending us off thinking about how to do this with a messenger bag when what you really want to do it with is a colostomy bag. There's a pretty big difference there.
I am not allowed to discuss the actual project but am doing the best I can to explain the intent of the product.