I'm having a hard time with fiber, I actually reduced the fiber requirements of my nutrient profile to the RDA. I don't really want to use flax because my recipe is already really high in isoflavones. While it's suggested by the research that a moderate dosage of isoflavones is beneficial to male (and female) reproductive health, I would hesitate to take that to an extreme when the consequences could be me enjoying Katy Perry, or worse.
If you have thoughts on the subject I welcome them. One idea is to eat some celery but that isn't very soylent-y.
Another tack to take is fixing the ratio of sugar to fiber, but I wouldn't know a healthy ratio from a deadly one. More than any thinking man that is.