Step 1: The strap
Wrap the pronged end around your messenger bag's strap then cut it leaving a good 4 inch tail. In order to keep the freshly cut end from fraying it's good to burn it. I used a "hot knife" which is the right tool for the job but scissors and a lighter work fine too.
Step 2: Sew the pronged end
Wrap the pronged end around the messenger bag strap and sew it to itself. The stronger the better, so many passes with the sewing machine is good.
Step 3: Size the second part of the strap
Step 4: Sew the second part of the new strap.
Step 5: Epoxy time!
I used some thin epoxy resin and "kleer kote" hardener. The ratio is 1:1, and since I only needed a little bit I used 4 grams of each which turned out to be perfect.
Mix it up and soak the "iffy" parts of your sweet sew job. Let it dry and then you have a super cool third-leg.