A simple carabiner from any dollar store add length the straps. Buy a quality one from a camping store for extra strength.
In the spirit of K.I.S.S. I've tried to keep the straps of the TCB's One Size Fits Most. And indeed they do. When the bag is laid flat and the top is opened completely, the strap lengths are cut as follows:
Small TCB: 54" (or about 13" exposed length)
Large TCB: 56" (or about 14" exposed length)
It's better that they're a bit shorter than longer, because it's a real pain to make them shorter later (the join and strap anchoring stitches have to be in just the right place, ends cauterized with a flame and stitched etc etc - all by hand).
But if you want to make them longer, it's simple. Buy an inexpensive, stylish black carabiner like the ones sold at the store I worked at*. Or you can get a real el cheapo kind from $2 stores.
You simply insert the carabiner between the D-ring and snap hook at the bottom corner of each bag. Viola!
* Read about my once-was part time gig at Eastern Mountain Sports
Have you seen the neat video of a TCB Customer Preston Tyree and his "Traffic Cone iBag"? Take a look - and join the TCB Facebook Fan Page just for the hill of it. http://www.facebook.com/trafficconebag