I often travel with Ryanair/Wizz Air and have a piece of luggage that fits their limits. However it's awkwardly constructed and a lot of the usable space is filled by the wheels and the handles.
What hard-shell model (currently available on the market) has the largest volume usable for storage and is still accepted on Ryanair's flights as hand luggage?
To quote Ryanair:
You can carry one cabin bag weighing up to 10 kg with maximum dimensions of 55cm x 40cm x 20cm.
The hand luggage should be available for sale in Europe. I assume a carton box (totalling 44L of storage) is technically the most optimal piece of luggage, but it's not exactly the most convenient thing to haul around.