Where you thinking do it yourself or gunsmith?
Williams Gun Sight Company make a GRS that look to be low profile and unlike most the rear sight is barrel mounted and the front a FO.
Wilson combat make a compact set up for the 870/hp9. "Trak-lock" I think it's called,seems to be a standard for the 870 type SG.
ETA;for the Williams ,looks like you might need Rifle sights to mount it on.