Ok... Here is how it was done in the trade show building business I used to work in. The rule of thumb is that the distance between the bulbs should be equal to the distance from the duratrans. If the box you build is 6" deep and after setting the light fixtures in the box, the bulbs end up 4" from the duratrans, then the bulbs need to be 4" apart from each other. This means that usually you will use single tube light fixtures which will allow you the ability to place them at the proper spacing.
The main thing is the light footprints of the bulbs should barely overlap each other so you have neither a hot spot nor a dim spot between the bulbs.
As to whether you need a frosted plexi behind...not necessary as the duratrans is made to act as its own diffuser. You could either sandwich the duratrans between to clear pieces or a clear front and a frosted (meaning translucent) plexi behind. For obvious reasons, white plexi won't work as it is an opaque plexi.
Forget using the pans from drop ceilings. Just look up at one and you will see that the light is not an even block of light but strips of light based on how many bulbs are in there.
Forget using the"diamond" plexi diffuser stuff. It doesn't diffuse the light evenly..you will see spots in the image.
Before you go ahead and have the duratrans made, figure out the approx. size you are going for and then go see if you can buy the fixtures that will allow you to fill the box with light.
Don't forget to put holes in the bottom of the box and a coupe of whisper fans in the top to keep the whole thing cool.
If you have any questions about what I said, please feel free to ask away.
I used to help design and build trade show booths and museum exhibits and the like for about 10 years.
Of course, if you have alot of bucks to spend, you should check out the Ceelite product.