Interfaces between immiscible polymers are widely known to be the origin of premature failure in multilayers and blends. Developing a small-scale, continuous method to fabricate interfaces from polymer melts with short contact times to simulate industrial coextrusion is critical for assessing new materials, polymeric systems, and processing conditions. Here we develop a miniature coextrusion die that can be fitted to a twin bore capillary rheometer for precise temperature and contact time control from 4.4 to 440 s. The design of the feedblock, stacker, manifold, and land zones was optimized via flow simulation software to allow bilayer coextrusion with minimal (<10%) edge distortions. Bilayer films of isotactic polypropylene (iPP) and polyethylene (PE) were produced using 90% less material than a lab-scale continuous multilayer coextrusion device, and the interfacial adhesion was in excellent agreement with iPP/PE bilayers produced using a continuous coextrusion technique for a range of PE molecular architectures. Finally, poly(ethylene terephthalate) (PET) and maleic anhydride grafted PE were coextruded. With a melt interfacial contact time of 22 s, interfacial adhesion between functionalized PE (both 0.8 and 1.5 wt %) and PET increased at least 30-fold over nonfunctionalized PE and PET. Using a moderate anhydride grafting density (0.8 wt %), interfacial adhesion also increased by a factor of 10 when interfacial contact time was increased from 4.4 to 22 s. The die designed here represents a convenient and accessible method to screen interfacial adhesion using less than 50 g of material, ideal for assessing new polymers synthesized at small scale.