Once, at SYD CX First-class check-in, I saw a man check a small bag of groceries. Not only was it accepted as hold-baggage (naturally, with its handles tied in a knot), but it was labeled with priority bag tags.

So, to add to the affimative chorus, based on this comparative, if not identical experience, yes, a cardboard box should be fine. Quite frankly, just plain-old bubble wrap might even get more gentle treatment for the lamp, as its fragility will be impossible to miss...