I ordered the large Kelty carport from REI last week (With my "dividend" check and the 20% off coupon it was about $50!). It should be coming into my local REI store tomorrow so I can play with is this weekend.
In looking at the pictures and playing with what I had in my shop I may have come up with a simple, non-damaging way to attach it to the rain gutters,
spring clamps. The ones I have are pretty strong and the rubber bits on the end keep them from scratching the paint or pulling loose. They seem to fasten to the rails really well. I'll let you know if it actually works.
I like the clamp solution because then I'll be able to put the awning on the van in a number of places, including the back.
--Pete