Thinest insulator between metal case and PCB?
(1 mm steel is thick!)

The isolation may not be required, since you're probably (S)ELV. Anyway, it's not going to cost you much space-wise. I wouldn't mess with paint sprays and such. Agreed, it's the thinnest, but I assume you can afford the thickness of a 0.1 mm PP (PolyPropylene, PP has very low water absorption) sheet? 

Try to use only SMT parts, and mount them single-sided. PTH components will add at least 2 mm because of the pins sticking out at the other side. A single-sided PCB may be glued directly onto the PP sheet, which in turn you glue to the bottom of the enclosure. If you manage to do the wiring of the PCB single-sided as well you don't even need the PP insulation. It may be worth using a couple of 0 &Omega; jumpers to ease the layout.  

You can save an extra couple tenths of mm by using a 0.8 mm PCB instead of the standard 1.6 mm. The thinner PCB is less stiff, but at the small size it's not a problem, and when glued against the enclosure it won't get any mechanical strain anyway.