I just used a set of PC speakers, pulled them out of their housing, then mounted them inside a 1.5" hole in my cabinet under my marquee...my cabinet looks more like a traditional cabinet where the marquee sticks out though. Judging by your drawings, you could do the same thing. Even if you didnt mount the speakers behind plexi, you could do a marquee frame like dandro drew to hold your plexi, and do speaker covers/grills for your speakers underneath.