Pack it in the original box.
Layer on a porno mag's worth of extra cover (or at least the escort ads from the back page of your local Sun/Now/Eye/etc...).
Then wrap it in post office brown paper (doesn't cost much for a tube of it). Two layers is nice...nothing will show through and an accidental scratch won't go through to the box.
Then tape it up (corners/seams and a couple of times around at either end of the box doesn't hurt).
It'll be a bitch to get into for the guy receiving it, but at least he'll get a chuckle when he gets past the brown paper.