Sounds like you want a dual-siderail rack.

My Tubus Cosmo mounts the panniers lower. On the lower rail, the rearward QL3 thingie fits nicely between a leg and the taillight mounting loop, so the pannier doesn't slide fore and aft on the rail. This also keeps the lower tab in place behind a leg. The taillight mounting bracket is nice for reflectors or battery lights even if you don't own a dynamo lighting system.

A $50 alternative is the Bontrager Backrack Deluxe (get the L size). I have one. It's a rack, nothing special. The panniers don't mount quite as low on it--by maybe an inch. I have the black one and have wrapped electrical tape around the lower siderail in two strategic places to keep the panniers from sliding around. When budget permits, I'll replace it with a Cosmo.