Single crystal alkali antimonide photocathodes have been shown to produce brighter beams than their polycrystalline counterparts. These single crystal semiconductors require a lattice matched substrate to be grown, but current INFN plugs lack the capability for this growth. To relieve this issue, we modified the INFN plug to hold a disk 1cm in diameter. This allows for studies of a wide range of advanced photocathodes and geometries on arbitrary substrates in high gradient photoinjectors. We show the modified design, analysis of the local field at the cathode and cavity detuning, and demonstrate the principle with a 1cm Yttrium disk.