You could get some dowels from Home Depot or Lowes, cut them off to length and sharpen one end. Then paint them to look old, rusty and glue them to the inside of your head trap.

Another more time consuming and but realistic way: Make a rubber/silicon mold of a large nail(s) and then cast the nails with latex. Then paint to your liking.

Yes they have mats that trigger things when stepped on. If you do a search for "trigger mat" or something similar I am sure you will find what you are looking for.