saw wave/square wave - the filter gives this its hollowness but never hurts to make the sound more hollow (square wave) to begin with
adsr is 2-2-6-3 you could use a ADBDSR and set it as follows 2-2-2-2-6-3 which will cause it to knock again at the end of the breakpoint
filter cutoff about 30% resonance the same - maybe use the adsr/adbdsr to drive the filter cutoff as well to create a more motile sound
pitch it down an octave from the normal pitch level