How can I install a window air conditioner in a metal window frame?
Since it sounds like it might be difficult for you to get to a lumberyard to build your own solution, you might be better off using a pre-made product.  AC-Safe, makes Universal Air Conditioner Supports that might be useful in your situation.



The system requires you to screw the platform into the window sill (which may or may not be possible in your situation), then simply rests against the outside of the building to support the A\C unit.

I've also seen brackets that mount to the bottom of A\C unit itself, and offer support against the outside of the building (but I was unable to find an example online. I think Thermwell makes them).