I am happy to report that since a few hours I'm the owner of a zafu!

Well, how happy I am I will know after sitting quite a few times, I guess.
I've chosen a kapok zafu, the picture looked similar to the one Shingen recommended, but it's "world cup-style" - a nearly round, fluffy ball! It's got an opening, so I can do all the things suggested, like take out kapok.
Contrasted to my hard cell foam block it's wobbly. I need to have a look at Taigu's sitting instruction again.
I guess when used a few times and wedged under the butt correctly, the zafu will flatten.
Otherwise I'm the first Burmese simply butt-balancing on a football.
Thank you for your advice!
Gassho,
Danny