Well smuggling is easy in space travel.
It's about time something was easy. thermodynamics dealt the deathblow to my beloved space pirates. If you're going to have a role playing game in a relatively hard SF setting you need some form of criminal endeavors to generate stories unless you're going to go all evolved humans don't do criminal shit on me.
Space craft by their very design invite smuggling. Not people (usually) but any sort of remotely realistic space vehicle will have lots of hidey holes for contraband. Just look at the ISS.
Show that picture to a bunch of players and if they can't find a few dozen spots for a stash get new players. On an unrelated note 'puka' is submarine slang for such little hidey holes. Try taking your drug sniffing dog into a zero gee trans hab for a real laugh. More high tech ships are less cluttered of course as they put those wires, conduits and pukas behind panels which enterprising people can remove.
Of course the dedicated men of the Space Patrol will have many and numerous detectors for contraband materials. Some things just aren't easy to detect, stolen plans on a storage device for example. They look the same as a bunch of porn files. Sealing other materials or using stealth materials is also possible. Hey, crime is not easy. It takes as good a brain to make a million illegally as it does to make a million legally.
Hiding smuggled goods on your hull in magnetized or otherwise attached modules is an old dodge. A customs ship docking with you is probably going to give you a once over with a laser edge finder. Make the goods look like a sensor housing or some other bit of hull candy. Of course a patrol ship will likely notice if you have another redundant sensor. The logical solution is to replace the actual device with a stash housing. It's logical unless you NEED that sensor or device.
If business school is suddenly looking good to your player characters you're running the game right in my book.
But the really insidious part of smuggling is you don't need a ship per se. Look at the International Space Station. It regularly spews forth hordes of little bitty cube sats weighing about a kilogram. It must be pretty easy to stealth coat one of those puppies. Now picture a bunch of the little cubes being dumped and picked up by a ship that already went through customs and heading off int the Black. The ship takes its little cubes to its destination and releases them along its course at just the right time for another ship or station to intercept it.
It sounds reasonable to me. A ship is a big heat radiator full of things like engines and people and habitats. Not so a little cube of dope or whatever. You can probably radar proof it's already small cross section and make it even harder to detect.
You want to get the intercept velocities right. A kilogram of cocaine can kill you, especially if it hits you at 7 kilometers per second. This is what navigators refer to as monetizing their knowledge of orbital mechanics.
For a real customs nightmare throw in humans with cybernetics. Does your legal authority allow you to disassemble this person to check for contraband? It gets really exciting doing cavity searches when the person might have additional cavities installed and/or a chaingun.
Smuggling becomes harder and easier in a post scarcity economy. Three dimensional printing can make anything you have a plan for. But governments and corporations might restrict the designs available to the public or charge a stiff fee for using them. Carrying designs on a storage device hidden in a puka becomes very lucrative.
I'm sure I've missed some angles and may need correction on others. But the picture that emerges of crime on the final frontier is one of a lot of sneaking, fooling inspectors and finding a safe haven for your illegal operations. Space is hard, even on criminals.