Your seat pitch is 75cm, so above every set of three seats there is 75cm of locker space. Why not divide it into three numbered sections, each 25 cm wide, and deep and high enough that a case that fits in the cage will go in? Tell people that they can bring whatever they like on the plane, but they must either wear it, put it under the seat in front, or fit it in their individual overhead locker. Anything that won't fit will either be thrown away or put in the hold at ruinous cost. Everybody wins (or at least Ryanair and everyone who abides by the rules wins).