I always used the D5 - but it wasnt easy. Today - this deck would have a tougher time getting through security...
But - IMO - it was the only Sony portable (of that era) that had a stable transport.
Both the D3 and the D6 were notorious for azimuth drifts and shifts...although they are stealthy-er...
The Marantz group, while a bit bigger - are made of plastic - (I think) - perhaps some advantage in the sneak...
But in the day, - a camera strap, hanging the deck high in the small of your back would do the job.
A buxom woman can also be of assistience concealing the D5...