Hmmm... pretty interesting.
This is how I would do it... I would put an IG body on a 40mm base (Terminator), laying on his back, or even against a rock/wall. Cave a hole on his belly and instead of 'falling' guts cover for the stomach, glue some grenades in the hole, and few on one of his sides. Then use REAL sawing thread (it is cheap and you can get it super-thin) to have a line from the grenades to outside the body. Then use some Green Stuff to cover SOME of the grenades. You need to leave them visible so players understand the trap.
Then use some IG and zombie parts to represent stages of decomposition. I would definetly use a zombie head and put an IG helmet on it. Once finished, put an open (zombie) hand face up on his side. Then take the thread, tie it to a laspistol and glue the weapon in top of the hand.
When painting, do not be afraid of making the line obvious... IMHO, if you hide it to much, people won't get your booby trap! Good luck. Keep us posted!