Three seal designs were examined for testing the Exploration Upper Stage in a vacuum chamber diffuser: a rubber boot seal, expansion joint, and iris seal with actuator. Expansion joints can handle extreme temperatures and stresses but rubber boot seals had torn during previous testing. The iris seal required further research. Alternative sealing methods were reviewed, and expansion joints were found to be viable, but more study of iris seals is needed. The best option for reliable sealing during high-altitude testing was determined to be a custom expansion joint due to its ability to operate under extreme conditions and loads while translating with engine movement.