To use a Bunnings Special (read: cheapo) hole saw on metal you need a decent, but most importantly, a variable speed, drill. Do it slowly with plenty of pressure to grind it down, doing it fast just makes it skip about, and wear the saw teeth out and smoke a lot.
And after cutting my hole i broke out the ball hammer, and folded a lip over all the way around, so no sharp edges to damage the hosing. Oh and gave it a few coats of black spray paint too.
But next car I'm planning on getting a piping kit that folds back on itself, and goes up thru the stock location, I know the flow isn't as good but atleast it's 100% reversible.