If your seat (whether that’s a chair, bench, or cushions) is flat, then this will cause you to slump, with all of the problems that follow from slumping.
Your seat should have a slight forward angle, to allow you to be able to sit upright with no effort. As explained earlier, you can achieve a good angle when sitting on a chair by having a 1″ (2.5cm) wooden block under each of the back legs. Even books can be used.
If your chair or stool is too steeply angled, however, then this will tend to throw your weight forward and cause over-arching.
This principle also applies when sitting on cushions, although of course you won’t want to put wooden blocks or books under your zafu. If you sit towards the back of your cushion, your pelvis will tilt backwards, and you will end up slumping. In order to have a slight forward tilt to your cushion, your weight needs to be to the front of the cushion.
It’s ideal if you can actually try out a stool for a while. Once you’ve done a few 30-minute meditations on a stool, you have a good idea whether it’s right for you. However, I must say that if you do an intensive meditation retreat (by which I mean at least seven to eight hours of meditation per day) a stool that you thought was comfortable under less demanding circumstances may reveal its limitations.
An excellent solution to help you find the right angle on your seat is to use a meditation stool with rounded legs, as in the one illustrated. When you sit on such a stool it automatically adjusts to the right angle for your body size, although you may still have to play around with cushions between your bottom and the stool in order to get the right height.
An even better solution, although a more expensive one, is an adjustable-height meditation stool. I often used to use one that had adjustable feet at the front and back of each side of the stool. I could therefore adjust both the height and tilt of the seat. The feet locked in place using thumb-nuts, and no tools were required to make the adjustment. I used that stool for my daily practice and it was generally very comfortable.
More recently I’ve moved to using a Kindseat, which also adjusts in height. The tilt of the Kindseat’s seat adjust automatically to provide the ideal angle for your posture. What’s more, the angle adjusts as you’re sitting on it! If when you’re sitting you move your knees apart or together by even a small amount, this changes the angle of your pelvis, and what was the ideal tilt to your seat is no longer ideal. The Kindseat takes care of that, because the part you’re sitting on adjusts with you. If that makes the Kindseat seem unstable, them I’ve given the wrong impression. It’s a very stable — yet responsive — form of meditation bench. I can’t praise it highly enough.
You can make a basic meditation stool using these instructions (PDF format).