Widespread adoption of biometric authentication technology has been slow because of technical, usability, and acceptability issues. Incorporating feedback from stakeholders throughout the design process is one way to ensure that biometric technologies have the best chance of achieving success. Emerging technology that performs biometric identification through unique eye movement characteristics may offer advantages, but the potential usability and acceptability of interfaces for eye movement biometric authentication is unknown. We explore a series of user interface designs for a simulated eye movement biometric authentication system, presenting an account of our design process, descriptions of the interface designs, and our evaluation. Our findings highlight the importance of feedback design, potential usability issues, users' perceived security benefits, and reasons why users may prefer eye movement biometrics over other systems.