A piece of 3/4 by 3/4 or wood similar will do. Just drill the size of hole you require through the short length of wood then cut the wood to just a "tad" shorter (the thickness of the tunnel steel, probably 16 guage) than the drill bit length.
Fine adjusting by moving the drill bit further in or out of the chuck. You should be able to just get the point to poke through.
You still need to check for wires, cables etc.
Happy Miles
Bryan.
