Shawn, your problem with that stuff is it'd probably be too rubbery when cured to sand easy.
The stuff I've used for this before is a variant of the blue PVC foam you can find at Home Depot (used for wall insulation). It can be worked pretty much like wood, except it's messier and less dense/easier to cut/sand.
If you get sheets of that, you can cut into pieces and then glue together with white glue to laminate into blocks. Then you attack it with a sander.