My back and my knees are why I switched to a bench. Sitting Burmese-style was causing some veins to burst near my knees, and the seiza bench seems to have cured that issue for now.
I posted up a thread showing pics of the bench I made for inspiration:
viewtopic.php?f=9&t=2433
As for making a zafu, I've made quite a few of those. Unless you are using buckwheat, you have to stuff them with filling until you think no more can possible fit, then stuff with a bit more. This goes for both kapok and cotton stuffing. If you don't, within a few days the zafu will be a limp, unsupportive throw pillow type thing!
Gassho,
Joshin