A trimless carpet base insert device, further provided with novel positioning and assembly features, enabling assembly and use thereof to be made in situ, by initially interlocking end members of the insert device with apertured portions of the cable raceway, thereby providing registration guide means for receiving and registering body portions of the insert device with said raceways, complementary interlocking means being provided on the body portion and end members. Thus the insert device body parts may be readily and compactly packaged and shipped and furnished to contractors and others in parts for ready assembly and installation in original building construction procedure or for subsequently positioning them as modules or units in openings formed in floors or other structural members.