You can try to find a panel that connects both paths coming from the top of the panel with the flag on it, but there will be two probable solutions if you do that. That won't help you to easily narrow your options down to the correct answer.
Think about the other sides. Is there anywhere you can narrow your options down to one panel?
UK Version
You can start by looking for the panel that connects the paths coming from the top of the panel with the flag, but you'll find yourself with two possibilities.
Think about the other sides and how they'll connect up. Is there anywhere that can help you narrow it down to one panel?
US Version
There's only one panel that fits the paths coming from the top of the panel with the car on it.
Think about how your example cubic maze would look when completed. Which parts connect where?
UK Version
Out of the two possible panels that you identified in Hint 1, only one fits the paths coming from the top of the panel with the car on it.
Think about how your example cubic maze would look when completed. Which parts connect where?
See the space immediately above the flag panel? The paths on the left side of the panel you place there should connect with the top of the car panel.
There are two paths running from the top of the car panel, one on the left and one in the middle.
The panel initially placed at the top right is the one that you should place above the flag panel. Once you put that into place, the rest is simple.
Look carefully at the parts that are directly connected, and arrange the panels so that the paths line up.
Solution
Incorrect
Too bad.
Are there any unconnected paths lurking somewhere?
Correct
Correct!
Now he can make a beautiful cubic maze using your example!