From the AAIB report on THIS incident
The fan cowl doors can be propped open by two stays
mounted on the inside of each door, to allow access
for servicing. When the doors are lowered from the
propped‑open position, a ‘hold open’ device on the bottom
of the fixed engine inlet cowl prevents the fan cowl doors
from closing fully under gravity. In this condition the fan
cowl doors stand slightly proud of the nacelle (Figure 1),
to provide a visual cue that the doors are not latched
So, they do not hang flush when unlatched, and given it's getting light before 0500, and departure was after 08, the walk round should have been in broad daylight. The doors stuck out a little, and the latches were hanging down, but for whatever reason, it was missed.
Not practical to make 2 engineers sign off closing the cowls, there are not always 2 engineers at outstations or at overnight stops.
Painting the edge a good bright reflective colour, or a length of reflective tape on the inside might make the gap stand out more.
Switches that tie in to the ECAS in the same way as cabin, cargo and emergency doors might be a way to go.
This is the first time an in flight fire has happened as a result of cowls departing, but there have been over 30 previous incidents, so this is not as rare as some might think, and needs a proper fix, not just a gentle reminder to do the pre flight properly, as that clearly doesn't work all the time.
As with the 777, this could have been a LOT worse than it eventually was. For that we should all be very thankful.