if no mounting holes at the rear end, nor near seat, P-clamps are the way to go, extra large ones that can be formed around the square frame bits.
Make sure the lower ones are good and strong, as they will taking most of the weight of panniers.
You can buy seat clamps that have two bolt holes for the horizontal parts of rear racks, but your frame may have an odd sized or shaped seat clamp area, who knows. check it out, if not, c clamps will work.
bike stores will have rubber covered c clamps and should be able to help you out