As I mentioned, I went with 1/8" white plexiglass, with a transmission rating of 2447, which, at 1/8" allows somewhere between 50% and 60% of light thru. I was going to go with 1/4", however it only allowed 35% of light thru and did not look very 'bright' at the plexiglass store. Plus, there is no 'load' on the diffuser, so 1/8" should not 'break' due to some unforeseen load. And, even at 1/8" thickness and 6" of separation, there is *no* appearance of 'individual bulbs' behind the diffuser.

This photo is of the new cabinet with only the diffuser and the bulbs on. In 'real-life' it is very bright and consistently lit across the entire surface.