Very cool post!

I've made my own seiza bench similar to this. My suggestion after iterating on this a little: Experiment a little and find your ideal measurements. Just screw but don't glue the legs until you find your ideal. This makes it easier to do modifications. This is not a one-size-fits-all situation, so if you're going to go through the effort to make your own, make sure it's a comfortable fit. Also, consider building it "pi-style" where the two bench legs are near the center of the bench and your legs go on either side. I find this less restrictive and easier to use non-disruptively on retreat. Doesn't seem to affect the balance negatively for me at all.