I used point source LED's, because that's what I had in my parts bin. This caused hot spots in the display. If you like it that way, you don't have modify them. I wanted more even lighting. The two most common ways of converting point source LED's to diffused is to either sand them or just encase them in hot melt glue. I decided to use hot melt glue. Just put a big blob of hot glue on each LED. I found that it looks more even if you can keep the glue blob smooth. The shape doesn't have to be perfectly round, but if you apply it in layers, the edges between the layers tend to cause variations in the brightness. In the photo below, the LED in the top right cell has hot glue on it, while the rest are bare. Note that it does a decent job of evening out the light.
At first, I used white copier paper to cover the LED's. It darkened the display to bit too much, and looked grainy. Then I went to an art store, and found .005" thick matte drafting film. It looks a lot better. I cut a 10 3/8 x 10 3/8" piece and glued it to the front.