We study, fabricate and characterize an all-solid polymer composite waveguide consisting of a multicore fiber for single-mode operation down to the visible. The individual cores of the multicore structure are arranged such that they strongly interact, to form the composite core. The behavior and parameters of the multicore geometry are analyzed in order to achieve true single-mode operation. The composite core fiber is fabricated with off-the-shelf poly-methyl-methacrylate (PMMA) and Zeonex 480R polymers and characterized.