The Gnomish Lexicon of the 1910s had an adjective G. hadra “opposing, facing, opposite” with an adverbial form G. hadron “opposite” usable as preposition in combination with the ablative, both based on the preposition G. had meaning “opposite” or in combination with the allative meaning “against” (GL/47). Tolkien indicated the adjective hadra also meant “equivalent”, though how or in what context isn’t clear.

Neo-Sindarin: For purposes of Neo-Sindarin I would update this adjective to ᴺS. dadhren “opposing, facing, opposite” based on the later preposition S. dan “back to, against”.