Hi! Let me show you my model of a lightbox of Ronaldo. I don´t have option of multicolor printing so I made the lightbox slightly different. 
The bottom part is where you stick LED lights. It has a hole for the cabel. Then you print the top part. You can print it by 2 colors (You pause the print, change the filament and continue). Also you can print it simply white and the rest you can paint black with some acrylic paint like I do sometimes as well. 
After you have everything done, you glue it together and tadaaaaa!

The bottom should be at least 20mm tall. 
The top maximum like 2 mm so the light goes through :)
The complete size I usually do around 20cm

Hope you like it and have a wonderful day!