sitting posture: better off without zabuton, the physics of it

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Zafu - Wikipedia:

A zafu is a seat stuffed with the fluffy, soft, downy fibres of the disintegrating reedmace seed heads. ... In western terms, colloquially speaking, zafu refers to a meditation cushion, and zabuton refers to the cushioned mat upon which a zafu is placed.

zabuton is a bad idea

If you sit full lotus, you don't even need a zabuton,  because you almost have all padded muscle and fat pushing the ground. All you need is some insulating material if the ground is cold. 

The problem with zabutons is they tend to be too soft and cushy. 

Here's an example so you understand the problem. Try to sit your normal cross legged posture on a soft mattress bed. Compared to sitting on the flat ground, it's very uncomfortable after a while. Why? Because the soft cushioning takes away the support that would otherwise maintain a 90 degree angle between your back and legs, compared to ground sitting. The softer the mattress, the greater the decrease in angle from 90 deg. 

Sitting on a zabuton is the same problem as sitting on soft bed mattress, just lesser in degree. It decreases the angle from 90deg, which will fatigue your lower back faster. Your lower back and core will have to strain to maintain a straightened body/back in meditation. 

most zafus are a bad idea

especially the circular one above, it doesn't utilize very much square footage from your potential full base.
see article

The Physics of sitting comfortably

If you must get a zafu, here's how to do it properly

For the cushion, your best bet is going to a crescent shaped one, to maximize square footage of butt and leg support, with buckwheat hull filling. 

The cushion in the picture above looks like it was predesigned to have a smooth decline. In the past, with other brands I've used, I had to adjust the amount of filling in the cushion to achieve the same effect.

When you first get the cushion, you can start by removing about one third up to one half of the hull filling. You can refill or remove more, by trial and error over time.

Before each sit, you shake and fluff the cushion into an incline wedge shape, so it elevates your spine and there's a gradual transition to the knees at a lower height. By default, a new cushion is stuffed to overcapacity, so that it's not possible to fluff it into an inclining wedge shape, with the result that the transition edge, at the edge of the cushion, creates pressure points on your legs/thighs. If you adjust the amount of buckwheat hull filling correctly, you shouldn't feel pressure points, it should be even support from knees to butt to spine. The recipe for comfortable long sits.