For replacing foam, how about using yoga or camping mat foam (as suggested earlier) and shaping it with a hot wire cutter?

http://www.vatsaas.org/rtv/construct...ireCutter.aspx

You could move the cutter along a track, and place the saddle/seatpost in an adjustable angle clamp. Then by checking and recording the alignment of saddle to cutter you could create the same angles for both sides and for future saddles.

For applying the foam, maybe use a heat gun to soften it then press the saddle base into it. Maybe set it on something that would shift/deform to help hold the shape (I'm thinking sand).