Had a similar situation recently, customer was very concerned about trenching as he has "spent years getting the flower beds to this level" and neighbour wasn't keen on having swa fixed to his fence.
So I got 2x2 tantalised timber, cut into 3' posts and tapered at one end, we drove them into the ground tight to the gravel boards at exact centres of the fence posts (between the posts) and fixed decking boards side on at the same height as the fence panel bottom rail.
So unless your right up against them it appears to be fixed to the fence directly, however my method appeased both the customers worry a and that of his neighbour.