What exactly do you mean when you say "modify zazen to suit yourself"? If your talking about posture, I would say yes, if you cannot sit in the usual full or half lotus postures, then you must find a stable, upright posture of some sort that you can maintain for a period of time. If you modify the actual practices to 'suit yourself' (I read suit your ego here) then I dont know if you can be said to be doing real zen training.
Both shikantaza and (especially) koan practice require a teacher.
I dont know if zazen is meant to be a relaxing practice. A 7 day sesshin is anything but relaxing, at least during the retreat.
Not trying to be judgemental or argumentative, just sharing my experience with practicing zen.