Generally, embodiments of the invention can include a linear shaped charge (LSC) end cap coupling structure adapted for holding an initiator structure adapted to initiate a booster explosive material, the booster explosive material, and the LSC in abutting contact with each other. One embodiment includes a rubber body formed with cavities adapted to receive the LSC, booster, and initiator structure (e.g., detonation cord). One internal cavity can be formed with a plurality of flexible protrusions or fins which are oriented towards a center axis of the preferred embodiment of three cavities configured to impart an interference fit with the initiator structure. Methods related to the invention are also provided.