Ok, looking for an update. Did some research and thread necromancy here seems the better option.
I need to store a small backpack content (<15 Litres) for 3½ days in early April.
Coin Operated Lockers - Narita Airport Guide
Baggage Storage - Narita Airport Guide
I see there is baggage storage available from ¥500 per day and lockers from ¥300 - so looking at the lockers.
How easy are the lockers to operate (Suica? Coin? Cash? Credit?) and how hard is it to find an empty one?
(I see 8 days maximum for locker storage.)