I had been looking around and collecting different kinds of foam samples to mock up and test the seat with. After some trials I found that a base layer of rebond foam with another layer of firm weight upholstery grade open cell foam over it provides enough bottoming protection from the rebond and enough cushion from the open cell. Here's a view of the bare seat pan.
These are the things I used on the foam. An angle grinder with an 80 grit flapper disk, some coarse sand paper and a block of wood, a hack saw with a 32T blade for cutting the foam into shapes, some 3M Super 77 adhesive spray, and some Three Bond 1521 adhesive. I didn't really use the Three Bond 1521 though.
I just had some strips of rebond foam, so I cut three pieces with the hacksaw and glued them onto the pan with some Super 77 adhesive. I put a slightly thicker piece at the back with the idea of making a kind of stepped profile. I used the angle grinder to flatten the surface and shape the step at the rear. The angle grinder and flapper disk worked really well at shaping the foam, but it was touchy to control. If the disk was not close to flat it would grab into the foam and remove too much.
After I got the base layer shaped I added a layer of firm weight open cell made from three strips.
Then used the angle grinder to flatten the top and shape the sides to match the contour of the pan sides.
To finish and level the foam I used the sand paper and block of wood. The block sanding worked well to level the surface, but tended to round the corners of the foam as the foam deflected under the sand paper. This rounding didn't happen with the angle grinder. I could get a really sharp corner with it, but found I couldn't control it well enough to smooth a large surface.
This view shows the layers and how I shaped the sides to match the seat pan.
I had read a write-up on seat making that recommended having a final layer of soft foam on the seat, so I added a layer of this pink soft weight open cell foam.
Here's how it looked on the bike. As can be seen the soft outer foam makes all the corners rounded, and when I tried wrapping some fabric around it to mock a seat cover I found it to look like cheap over-stuffed furniture.
I tried pealing, shaving, cutting, scraping, but the only way I could remove the outer foam was with a wire wheel mounted in a hand drill. The result on the floor was not pretty. Heres the stripped seat. I made a few gouges at first until I learned to control the wire wheel.
I went over it with the flapper disk and sanding block to get the remaining glue off. There is one remaning gouge I need to fill before its ready to cover.