Setting aside the feasibility of solar sail propulsion, power will be an enemy: just getting a typical cubesat level of 5W is much harder at Neptune, since Nept

Setting aside the feasibility of solar sail propulsion, power will be an enemy: just getting a typical cubesat level of 5W is much harder at Neptune, since Neptune gets just 1/900th the solar flux. By way of comparison: the "Juno" probe produces 486W at Jupiter (solar flux 1/27th of Earth's) from about 72m^2 of solar panels, weighing 340kg. So if similarly scaled 5W at Neptune is about 110kg + 24m^2 of solar panel. And that's assuming that 5W suffices: "New Horizons" has 200W and a 2 metre diameter radio dish, and still took 15 months to download the data from its brief Pluto encounter (although at a slightly greater distance than Neptune's orbit) If you want it to be an orbiter rather than a brief flyby then it needs some way to shed a lot of velocity: either a lengthy rocket burn and/or aerobraking by grazing Neptune's atmosphere. So unfortunately it seems well beyond cubesat terrain: even a "cheap" probe to the outer planets is a large and complex machine.