Ok, so I work with a few upholsterers so I guess you could say I'm in the game. The foam you need for this sort of thing is high density chip foam. The padding can be made more comfortable by laminating a softer "blue" foam over the top, thus giving you the density(which also helps keep the padding in shape) and the comfort. On the Shimano one there are two pegging point, one on either side at the front, with which you can peg the seat to the floor, this means you can lean back into it a little more and also stops the seat getting blown into the drink when you are not seated in it. I strongly suggest adding the pockets as they are useful, and you can certainly get enough terminal tackle, camera and scales etc in the pockets for a short session.
Personally I'm amazed that all the big boys have discontinued their stalkers, if only one(a decent one at that) had continued I'm sure they'd have sold well.
Haydn
P.S I may also be able to help with the foam suppliers depending on where you are. P.M me if you need it.