How hard would it be to add a shoulder strap to this sort of bag:

http://www.redoxx.com/product_categories/8/products/22-small-aviator-bag

and how would i go about doing something like this?

Because the zippers go down all the way, I was thinking of having the strap attach right next to the zipper, not over it. And run it diagonally to the other side so the bag doesn't sag one way when worn using the strap.

Another option would be to have the webbing cover the zippers about half way up from the bottom on each side (I really won't need to zip it all the way down). That way, the strap will run down the middle, and the bag won't sag.

That bag is really cheap compared to it's equivalent (in size) with a shoulder strap for $175
http://www.redoxx.com/product_categories/3/products/16-flying-boxcar

Yes diagonal would be best make sure to stitch the strap to the bag securely. An X with a box around it is the norm.

-N-

Awesome. Would you know of any suppliers I might be able to get hold of the material from?

Another idea I just had is something like the x with the box on both sides of the zipper, running and making an upside down Y. From there, a single strap going to the other end with similar setup.

Like this:

http://img172.imageshack.us/img172/9249/aviatorsmall91051sm1ja6.jpg

The thrift stores in my area have tons of old bags for cheap . . . pick one out that works, get a stitch-ripping tool and remove from old, attach to new. Otherwise I would look at a camping store in person or online to find that sturdy woven strap material . . .

Awesome. Would you know of any suppliers I might be able to get hold of the material from?

Another idea I just had is something like the x with the box on both sides of the zipper, running and making an upside down Y. From there, a single strap going to the other end with similar setup.

Like this:

http://img172.imageshack.us/img172/9249/aviatorsmall91051sm1ja6.jpg

That'll do quite well. Just remember that the top of the ^ where everything joins is the point that needs to be the strongest.

-N-