A test bench for Kaplan and Francis pico-turbines was developed at the University of Applied Sciences of Fribourg (Switzerland) to test different axial turbine designs from 0.3 to 6 kW. Using the experimental results, this test bench allows comparing the results of the CFD simulations and of the measurements of the tested turbines. During the test rig design, due to the different specific components of the axial turbine, a new concept was developed to replace the spiral casing. The current paper presents the design of a substitution solution for the spiral casing of pico-hydroelectric plants and its performance compared to the standard solution by numerical simulations. The new concept, called "water-box", had to fulfil some criteria: allow adapting to different flow conditions (flow rate up to 65 l/s and to a pressure range up to 10 bar), while ensuring the same uniform flow distribution as with a standard spiral casing. In the same time, it had to allow optimizing other parameters like the space needed and the costs. The idea is to keep the principle of a sideway inlet and an axial outlet but using standard elements of industrial piping to reduce the costs. With this geometry, the fluid is naturally rotated and pushed towards the axial exit. There, a convergent pipe is mounted and finally, a stabilization grid which homogenizes the flow and break the rotation before the turbine. The performances of the new concept were evaluated and the results compared with a "standard" designed spiral casing by CFD simulations. The results showed that with the "water-box", the pressure losses are 22 times higher. Nevertheless, the manufacturing simplicity of the water-box compared to the spiral casing achieves a substantial costs reduction of about 65 %. Thus, all the criteria were fulfilled. This simplicity associated with performances, which remain very interesting open new fields of application notably for pico-hydroelectric turbines installed for example on domestic sites.