I'd been playing around with chords on the bass guitar and this morning settled on the key F#.
This is the third take, so it wasn't entirely improvised but I wasn't at the point of following an established song yet.
In Ableton Live I added a couple of reverbs, one to mimic a room to fatten the tone and another for the high-pitched ambience. I also used EQs to filter the lower frequencies, which can be overpowering even when playing higher notes.