A non-conductive composite insert is provided between conductive portions, useful, for example, in downhole EM telemetry applications as an external “gap sub” in a drill collar, or as a sonde-based internal gap. In a preferred embodiment, the composite is made from a glass-fiber reinforced plastic, and separates non-magnetic conductive portions made from stainless steel. The composite insert provides a slanted or tapered transition into the conductive portions at either or both ends of the insert. The transitions on the composite insert may comprise one or more tapered surfaces, which may be male or female in configuration with respect to matching transitions on the conductive portions. The transitions may be bonded together by adhesive, or alternatively may be threaded.