I'm trying to fix a door bolt to the inside of a hollow metal patio door.
The door is made of two sheets of metal with a cavity between ( which may have foam in it or some soft insulating material.
A toggle or molly bolt would require a big hole and probably isn't suitable because the metal sheet is too thin. Most of these fixings need a 6mm minimum thickness and are therefore only suitable for plasterboard.
I've thought about going right through both leafs with a machine screw ( I fixed a door knocker to my front door this way and it worked well ) and having some kind of shear nut on the outside but I'm worried about the security implications of this and can't find a suitable shear nut for an M5 screw either.
The total thickness of the door is 45mm and another 15mm is needed for the bolt so to go right through the screw would have to be 60mm plus.
Any ideas ?