whoa!  i didnt know you were building a showcase.  well i think i did actually...just never saw pics of it.  cool looking cab...very unique.  
when you say route out a slot...i was thinking of doing this, but i cant find a 1/8" thin slot bit for my router, and im afraid if i use what i have now, its like a 1/4" thick slot cutting bit, it will be too much space and the marquee/plexi will not be secure...more wobbly and stuff.  what are you going to use?