It's illegal to search a vehicle without probable cause or consent of the vehicle's owner. A cop can look in the windows though and see something that can give him PC to search the vehicle. So yeah, hiding your boxes or putting a blanket over them or something is likely sufficient. That said, because a car is mobile, an officer does not need a warrant to search. Just PC. So if they have PC for any other reason (say, canine unit barks, or perhaps they smell an illegal substance, etc.) they can search.
Now I ain't trying to see no highway chase with Jake
Plus I got a few dollars I could fight the case
So I, pull over to the side of the road
I heard "Son, do you know why I'm stopping you for?"
Cause I'm young and I'm black and my hat's real low
Or do I look like a mind reader, sir? I don't know
Am I under arrest or should I guess some mo?
"Well you was doing fifty-five in the fifty-four", uh huh
"License and registration and step out of the car
"Are you carrying a weapon on you, I know a lot of you are"
I ain't stepping out of ****, all my papers legit
"Well do you mind if I look around the car a little bit?"
Well my glove compartment is locked, so is the trunk and the back
And I know my rights so you goin' need a warrant for that
"Aren't you sharp as a tack? You some type of lawyer or something?
"Somebody important or something?"
Child, I ain't passed the bar, but I know a little bit
Enough that you won't illegally search my ****
"Well we'll see how smart you are when the K-9 come"