So, I have this messenger bag (Mark2) that I need to use as a scout bag on a camping trip that I'm going on soon. Since messenger bags have a tendency to get in the way when you don't want them to, I need to turn this thing into a rucksack but I don't want to loose the messenger bag quality....so...enter the Dr. Franken-bag (heretofore to be referred to as Mark4 (Mark3 was a complete bust and didn't make an Instructable about it, which is why you're asking yourself, "Wait '4', where's 3? What did he do for Mark 3?").
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Step 1: Build a New Strap
I got myself some rope (about 6 feet the thicker the more comfortable; this stuff is half-inch (about 13 mm for you forward thinking folks who decided base 10 make adding easy)). I measured it by wrapping it around my back and holding it out in front of my in both hands so it made a bit of a horseshoe shape around me.
I then ran one end through the D-ring on one side of the bag and tied a double overhand knot to secure it as a sliding knot. This will allow me to shorten or lengthen the strap as needed. Sort of like, it's technical, but it uses Transformers-like technology to accomplish that part. Luckily, the patent ran out on that like 500 years ago, now all ropes can utilize sliding knots license free. WOOT PUBLIC DOMAN!!!
Lather, rinse, repeat for the other side.
Step 2: Hang on to Your Nickers....
Now, slide the strap over your head and under your arms; be sure to have the front of the bag facing you. With a mighty YAWP and BY THE POWER OF
GRAYSTOKE GRAYSKULL flip it over your head...It'll land on your back and auto-magically make shoulder straps! Don't ask me how; there's some sort of quantum interaction happening with an identical rucksack in a parallel universe (well, almost parallel, the colour mauve smells odd for some reason there, and I've never quite gotten used to the blue tonged skink there either). I don't know, it's kind of difficult to explain, something to do with a cat locked in a box and some prisoners being offered a plea bargain or something. Well, I said it was difficult to explain!
So, here's just a quick view of it on Youtube...I'd rather have an animated gif but my resources don't make them well (would appreciate some help from the internets on that).
And that's it. as you can see from the first pic at the beginning, I can still adjust the length of the strap to make it hang just like a regular messenger bag, but we all know it has a secret identity.
And now you're probably asking yourself, "Why didn't I take the blue pill when I saw this?" Well, don't ask me, I took two.
UPDATE: You might also find that the straps will stay on a little better if you put the bag on initially with the front facing away from you, and then twist it around to have the front facing you again, and then flip it over. The straps cross high on the shoulders and tend to pull the strap toward the spine.