Just wanted to pass this on and see if anyoone's tried it or if anyone has any feedback on it...
I'm sort of always on the go, and travel a bit. Through the suggestion of a friend, I've discovered that "Yoga Brick," a block of dense foam or cork about four inches thick by six inches wide makes a pretty passable and convenient travelling Zafu. Turned on edge, it also works pretty nicely for seiza. Not real pretty, not really traditional, probably non-Kosher to some elements, and probably not for "all-the-time" practice, but on the go, they're real small, real lightweight, and are actually quite comfortable. I'm thinking of adding a somewhat softer thin foam layer to the top, but otherwise, I sat tonight on a yoga block for about forty minutes with no problem whatsoever. Using my yoga mat doubled over as a makeshift Zabuton, I was all set! And the whole shebang is packable, travels light, waterproof, and comes in at less than $20 in all.