Rachel,
Have you tried your local Home Depot/Lowe's/Hardware store yet?  They have
some blue styrofoam-type stuff that is quite firm but is cut easily(?) with a
kitchen knife.  It comes in thin or thick sizes for you to be creative with.
Also, there are pink and gold colored foams, too, with different properties
and facings that you might try.  I would stay away from the white styrofoam,
it is very soft and walking on it would dent it or compress it.

Or, buy a couple gallons of Bondo and spackle the floor smooth!

Bill Marshall
85 GL Tiico "Pandora's Box"
Aurora, IL