A symbol set of N elements, wide and narrow, would work.
One of the things that makes regular morse hard to learn is the variable length.

Hell, that's how I send debug information from my devices, My "pong"
routine outputs 8 pulses, wide for 1 and narrow for 0. The total width
of any byte sent is always the same though, allowing me to stack up 50
bytes if I have to, and still read it with my scope's trigger offset.
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