Hey, I had a brainchild the other day while thinking about what kind of foam to use for my flat cafe seat. Also, since there's quite a few of us putting these together at the moment or near future, I thought I'd share my idea.
I purchased a couple of gardening pads - the ones people use to kneel on while weeding etc. They're rectangular in shape, quite firm, and are covered in a weatherproof rubberizing compound, and cost $4 each at Walmart. I haven't upholstered mine in vinyl yet, but my experimenting has been positive. Since I'm planning on having about an inch of foam, I was worried about using foam that was too soft. This stuff is supportive, but also very cushiony for my bum. It cuts easily, and looks like it will hold up well to useage. Also, I plan on making my cover removable so that if the foam does happen to wear out, $4 buys me a new seat.
Fight the good fight!
-Jay in Philly