John has a good point. For light credit-card touring, you don't even need rack-mount eyelets, and you can use a rack that just clamps onto a round aluminum seat post, or a handlebar bag, or even use a very large seat bag and no rack at all-- or some combination of these. The Mountain Wedge III seat bag from Jandd Mountaineering has about 450 cubic inches, and, by itself, seems to be just enough for a 3-day CC tour:
At the extreme, there are seat bags with over three times that much volume, kind of like having a duffel bag hanging back there, but it has internal metal supports to keep it from swinging and sagging:
These are the Carradice Camper Longflap (on the left) and Super C (on the right).
Having the weight up so high is not ideal for handling, but you adapt your handling method and then you don't think about it. I say that partly from experience, as the first picture above is of my own bike. Although not with a heavy load, I had it up to 53mph recently, and could have gone a little faster if there hadn't been any traffic. I couldn't feel the bag back there. A big seat bag will give you less wind resistance than panniers too.
Here's a seat-post-mount rack from Topeak, with a 20-pound load limit: