I just built mine this weekend.....as you can see in this picture, i had a little bit of hotspotting going on in the upper left hand corner, even after installing a diffuser panel ( 3 of them actually ). My solution was to paint one of the diffusers white with spray paint. Doing this still allowed for plenty of light to come through, and also made the lighting perfectly even!
I also setup my sign to be controllable from my remote control ( irule ). I can control, on/off, bright/dim.
all in all, a very cool low cost project!