The IAC in Tenerife is collaborating with an international consortium involving the University of Hawaii, MorphOptic (USA), Dynamical Intelligent Structures (DiSL, Canada) and two institutes in Lyon (CRAL@CNRS + INSA; France), to design the future $\geq$40m ExoLife Finder (ELF), an optical hybrid interferometer telescope. The objectives of ELF are both scientific and technological: the discovery of biomarkers in the atmospheres of exoplanets and patenting tensegrity and ultra-light ultra-thin mirrors to reduce the weight of future large telescopes by at least a factor of ten. The consortium embarked in building a 3.5-m prototype (nicknamed Small-ELF) as a demonstrator for ELF, made of 15 mirrors of 0.5m in diameter mounted on a "bicycle wheel" structure. Small-ELF will have high angular resolution astronomical capabilities in the near-infrared and will also serve as a technology demonstrator for the much larger ELF. The installation is planned in the coming years at Teide Observatory. The detailed design, which provides optical, mechanical, and electronic solutions with bottom-up cost estimates, is presented.