Growing multi-elemental complex-oxide structures using an MBE (Molecular Beam Epitaxy) technique requires precise control of each source flux. However, when the component elements have significantly different oxygen affinities, maintaining stable fluxes for easily oxidizing elements is challenging because of a source oxidation problem. Here, using Sr as a test source, we show that a crucible aperture insert scheme significantly reduces the source oxidation in an oxide-MBE environment. The crucible aperture insert was shaped like a disk with a hole at the center and was mounted inside the crucible; it blocks most of the oxygen species coming to the source, thus reducing the source oxidation. However, the depth of the aperture disk was critical for its performance; an ill-positioned aperture could make the flux stability even worse. With an optimally positioned aperture insert, the crucible exhibited more than four times improvement in Sr flux stability, compared to a conventional, non-apertured crucible.