Quick update: I've decided to keep the Lucas 21" Expandable (A Saks clone)

My solution is to adapt the bag to accept a standard strap (in this case the Tom Bihn or the Op Tech SOS) and ditch the cart.

Rather than permanently modify the bag, I'm going to attach removable hardware on the bag to add two D-Rings; this will give the needed connection points for any aftermarket strap. This technique will likely work on other bags, so even if you don't have an A Saks or Lucas, it could be helpful to you.

The product I'm going to use is similar to a locking carabiner (used in rock climbing), but in a different shape. Specifically, I was looking for a D- or triangle-shaped ring with a screw gate.

I found what I was looking for in this product: the Maillon Rapide Delta Quick Link. It's not sold at too many places online; I bought mine from Green Boat Stuff. The pieces I bought were $7.50 per unit (I'll use two on this bag), which was pricey. If they are made to be used as marine hardware, I figure they will last forever. I like the idea that I can remove them someday if a warranty issue comes up, or if I move to another bag.

I'll post pics on the bag when the hardware arrives.