What?

No, don't make the combos into the ffc's combo...

Right:
1) Compile the script
2) Set the script to a slot
3) Go to 'ffc 1' for the room you want the pits in
4) Set that script to the ffc, and set it's combo to a transparent combo that is not combo 0
5) Make all your pit combos into walkable centre statue combos
6) Place them on the screen
7) Open in Zelda Classic and play.