Revelate Designs Tangle frame bag. I put one on my fat bike because my spare tube was too large to fit into a saddlebag, and I didn't want to bring the pannier if I wasn't heading to work. Easy on, easy off, waterproof, and relatively innocuous looking, compared to trunk racks. Surprisingly roomy, as well; in mine I keep a spare tube, tire pump, tire levers, patch kit, large dial pressure gauge, multitool, crescent wrench, 4 large bungees, lightweight gloves, cheap sunglasses, and
a couple of tennis balls, plus I can fit my U-lock in there if I break it down first.
If you can't fit your clothes in the Tangle, Revelate also makes full sized frame bags. A little pricey, but the construction quality is top notch, and they're made in the US.