what's the best way to cut a flat surface without drilling a starter hole?

since your saw cut will make a gap (the kerf), get a small (1mm) drill then chain a series of holes together until there is enough to get the saw blade in.

Or

if you are fitting a hinge for your flap, hid the start hole under the hinge plate.

Or

take the metal to a local workshop equipped with a laser cutter, (look in yellow pages)

If the sheet metal is thin you can probably start the cut with a chisel

and finish with shears. A bit of careful hammer work on an anvil and

you almost won't see the 'start' mark.

Angle grinder, dremel tool, torch, plasma cutter.

It depends how thick the metal is. I would use a angle grinder for thick metal https://en.wikipedia.org/wiki/Angle_grinder or for thin metal a rotary grinder http://www.dremel.com/en-us/Tools/Pages/CategoryPr...

Use a knife to cutit