I doubt that filling a long thin channel will ever work. The heat + pressure will always collapse the thin infill walls.
In general infills does not provide much strength to a part, it is way better to have stronger walls.
And z-direction does not need to be more stable than the other directions so there is no need for long continuous strands anyway.
Maybe it would work better with smaller, less tall, slots at the inside of the walls.
Lets say 2-3 layer heights tall, continuously filled slots, which are then interleaved with each other. More like bricks less like columns. The outer wall layers would provide stability to prevent collapse. And over spill or bulging would occur towards the inside of the part.
In general infills does not provide much strength to a part, it is way better to have stronger walls.
And z-direction does not need to be more stable than the other directions so there is no need for long continuous strands anyway.
Maybe it would work better with smaller, less tall, slots at the inside of the walls.
Lets say 2-3 layer heights tall, continuously filled slots, which are then interleaved with each other. More like bricks less like columns. The outer wall layers would provide stability to prevent collapse. And over spill or bulging would occur towards the inside of the part.