Lots of people use clear mini blind wands/handles. You can stick an LED in either end and the whole thing will light up like fiber optic. They mostly use them as halos around projector headlights but if you could find one long enough I guess you could use it for the 3rd brake light.
The other option is to "frost" the lens so that it diffuses the light from each individual LED making it appear as one big light.
I like the individual LEDs because I incorporated sequential turn signals into my Esky brake light along with the brake light function.
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