A standoff attachment for bonded fixation to a substrate includes a standoff post with internal nut for engaging a mating fastener used to mount a support element such as a line clamp substantially at a distal end thereof. The standoff post extends from an enlarged bond-on base and defines a hollow slotted channel of noncircular cross-section which terminates at the distal end in an in-turned flange defining an open fastener-receiving port. The nut includes a keyed rib sized and shaped for axial slide-fit reception into the slotted channel at an inboard side of the distal end flange, wherein this rib mates with the slotted channel shape to preclude substantial nut rotation. An elongated slotted plug is secured within the slotted channel to retain the nut, preferably with a minor degree of axial freedom, axially between the plug and the distal end flange.