I didn't get a good result when trying to make a box for my Space Invaders either...
I planned to build it in two parts (following similar layout as Giacomo Vernoni with his sleeves, that I had bought for my other cartridges):
I sent it to print in 300g cardboard. After glueing it, result looked OK:
But, cardboard is not rigid enough, and Box looks too fragile. Additionally, this cardboard did 'suffer' when bended (specially the painted area):
Given that, I did a second try, printing in the same cardboard but, this time, I ordered it 'laminated'. Result, from a rigidness point of view was very good, but now I had the opposite problem. It was really dificult to bend it (even having marked the bending lines with a knife). Besides that, when bending, the laminate tended to detach:
Anyway, if you don't look at it too close, looks nice with my other cartridges...
Cheers