This report summarises the feasibility study result of AltiCube+, a low-cost long fixed-baseline radar altimeter solution based on CubeSats on-orbit assembly. Various altimeter concepts based on the AltiCube+ solution, including off-nadir SARIn and a Multiple-Input and Multiple-Output, have been explored, aiming at sub-kilometre cross-track resolution for water level monitoring. Key technologies on deployable structures, autonomous assembly and highly capable platform have been assessed. The final concept consists of five 16U CubeSats docking with each other to establish two side-looking interferometric altimeters, each with 6.6 meter baseline.