You might wish to look at http://www.performancebicycle.com They sell a range of racks and panniers. I have used their products for decades... they hold up well.

I have been using a Blackburn rack (also from Performance) on my commuter bike... it attaches to the seatstays up near the seatpost collar, and way down near the dropouts. I was lucky in that my bike's frame already had drilled & tapped holes for the rack. Otherwise, Blackburn supplies clamps that would appear to work just as well. It will accept panniers, but I don't use them.

Instead, I use a Performance Bicycle rack trunk. The trunk is very roomy, reasonably water proof, is covered in a retroreflective material that sparkles under headlights' beams, and, most importantly, was on sale! Inside, it is large enough to fit eight soda cans, and expands vertically to half again its normal height. It won't fit a laptop computer, so I lug that home in a backpack, on days that I need to.

I do have a set of wire & canvas grocery panniers that I used in Paris, and I'll attach those for the occasional non-motorized grocery run. It is perhaps a sad statement on the purchasing power of money, but I can easily fit $50 worth of supplies in those bags, and ride home, sans probleme.

I really enjoy the startled and confused look of the other shoppers as I load up & ride away.

Oh... Keep up that good work, in the face of your sedentary coworkers. If they ever ask "Why do you insist upon riding your bicycle to work and back?" Just reply: "Y'know how you arrive home after a bad day at work and a worse day on the freeway, frustrated, p*ssed off, and late, just mad at the world? Not me. I'm just happy to be home!"