one thing i've taken to doing occasionally when i want to smuggle items in the latter part of the alphabet (like synthetic marijuana, even though it's not that profitable) is piling up 1 unit each of a bunch of 'crap' to fill up my hold.
I.e. taking 1 unit each of basic alloy, beryllium, boron, cobalt, construction machinery, consumer goods, copper, diamonds, engine comps, fertilizer, food rations, gate/lane parts, gold, h-fuel, high-temp alloy, hydrocarbons, light arms, luxury consumer goods, mining machinery, mox, niobium, optical chips, optronics, oxygen, pharmaceuticals, polymers, scrap metal, ship hull panels, silver, super alloy, and superconductors.
Probably missed a few, but if you have a bunch of that crap in your cargo bay, since 'cargo scanning' works on an alphabetical sorting method... they have to scroll down through all of that junk just to see the synthMJ you're carrying.