An insulated, sealed cap overlies and protects a fastener component or the like on a substrate in association with a substrate opening. The sealed cap includes an outer cap component filled partially with a selected sealant material and assembled with an inner collet sized and shaped to fit with a slip fit about the fastener component such as a nut or the like at one side of a selected substrate, such as the skin of an aircraft. An inboard edge of the inner collet seats on and substantially seals with the substrate. The outer cap component is then displaced toward the substrate to extrude the sealant material into a thin and substantially uniform layer joined with an extruded outer bead on the substrate, and then permitted to cure. Tapered ribs on the inner collet effectively lock with the outer cap component during sealant material curing and subsequently.