I would like to recommend a book about, and entitled, "THE POSTURE OF MEDITATION" (by Will Johnson).
I believe that its philosophy of finding a sitting posture is very much as we encourage here at Treeleaf, namely, we each have to experiment with our own self and make small adjustments to find (within certain rules) the posture "right for my particular body" (one size does not fit all). Further, sitting is not rigid and fixed, but always subtly fluid and changing, such that the posture at the start of a sitting period will not be precisely the same as at the end (or on different days!).
For this reason, the author presents a philosophy of sitting, and a series of exercises, to help each of us find our "sweet spot" (again, a "sweet spot" that is not stagnant, but needs to flow and change even during one sitting period). It is based on finding (1) an alignment of the body (head, neck, spine, buttocks, legs) that is balanced and in line with gravity (2) relaxed, yet (3) resilient. YOU KNOW IT WHEN YOU FEEL IT. Better said ... when the body feels right, and when the body feels balanced and "drops from mind" (becomes no longer a distraction), it probably is right and balanced.
This is also very helpful to our situation here, sitting at a physical distance from each other, for I cannot reach out and adjust peoples' postures. For that reason, I recommend that people first seek some basic instruction at a local Zen center (or other Buddhist group) or, if not available, with a local Yoga teacher (very good option, actually), then experiment and adjust on one's own (when the body is forgotten, and no longer an issue, it is a good posture). HOWEVER, the reality is that, even at an "in the flesh, under a roof" Zen group ... it is not that different from Treeleaf because the teacher will usually show beginners the basic posture once or twice at the start, viewed from the teacher's position outside (not from within the student's body and actual sensations) ... then leave the student on her own most of the time. The student still has to "figure out sitting posture for herself".
Now, a few cautions on the book ...
I - The author does not emphasize any particular posture. While he does praise and encourage the Lotus Postures or Burmese (as do I), he also says his philosophy works well with a sitting bench, chair or the like. (Like me, he does say not to give up on trying the Lotus Postures too easily ... and to really give them the 'old college try' ... that westerners tend to not give it a sufficient attempt). What is more, there is very little specific advise in the book on how to sit ... apart from some basic rules (such as keeping the knees down on the floor if in Lotus or the like, and the pelvis higher than the knees) combined with his "self tests" for finding a balanced/relaxed/resilient place. So, for example, there is very little specific advise on how to get the knees down on the floor, what to do if the legs fall asleep, etc., except for his advise to keep adjusting until the problem resolves itself. (We have some other threads here at Treeleaf, by the way, on stretches and other strategies for those issues).
However, his general philosophy should help you find some answers that work on your own body.
II - He is a little too focused on keeping the body constantly in mind, and making constant adjustments. I would prefer (in keeping with our form of Shikantaza) that folks adjust the body to their "sweet spot" ... then forget about it, and drop the body from mind, returning to "just sitting" ... until a little later in the sitting, for example, when they might give themselves a little "readjustment" if they feel they need, returning to "just sitting" again. I would recommend to "adjust and drop the body from mind", instead of making it a constant object of focus and adjustment, as he sometimes recommends.
III - Some of his claims in the book are a little new agey and extreme ... such as that staying in line with gravity will have some effect on lengthening life expectancy. (Anyway, true or not ... a minor part of the book. And would it not be nice! ).
IV - Some of his recommendations in the last chapter for keeping a balanced/relaxed/resilient posture in all of daily life sound not very practical, and some may be downright dangerous (like his recommendation for doing so while driving a car).
However, with those few cautions ... I recommend the book to everyone at Treeleaf who would like some assistance in finding "the posture right for one's own body".