So-called "P-clamps" are useful for such a situation. A Google search will yield pictures, your local hardware or homeowner's store should have them on the plumbing or electrical section. Regarding the weight of your panniers, the attachments to the seat stays are mostly to keep the rack from rotating back and forth, the majority of the weight stresses are absorbed by the attachments to the fender eyelets at the bottom. I'd stay away from the seat post-type racks.