Multiport interferometers are an important tool in the emerging field of quantum information technologies. In theoretical work, we investigate implementing Haar-random unitary transformations in increasingly large interferometers with realistic imperfections. We find that random matrices result in mostly low values of interferometer beam splitter reflectivities. We model production imperfections and we find that these severely limit the implementation of random matrices. We show the effects of the imperfection can be mitigated through optimisation of interferometer degrees of freedom and by adding additional beam splitters. In experimental work, we investigate the realisation of reconfigurable multiport interferometers in silica-on-silicon integrated photonics chips using a modular design. We show that individual modules are fully reconfigurable. We give a proof-of-principle of the design by connecting three modules for the first time and measure 5% transmission.