This is what we used--you can get prop switchblades that when they open up they are actually combs---file off the teeth of the comb--then we piled law books on the table and attached a piece of styrofoam to the table with the books surrounding the foam.  When he threw the knife down it stuck in the styrofoam and protruded above the books.  We did this in a black box theater in the round with the audience only four or five feet away--and it worked really, really well.