I plan on doing mine like knievel did with his evolution. Basically there is a dado in the speaker panel piece that the glass/plexi slides into. You slide the glass up into this slot and push the bottom of it over some lip that holds it in place. This way the glass is removable yet there are no gaps in the top or the bottom.
As far as gaps on the sides, I'm not exactly sure how to fill those gaps but I'm hoping I can get my glass cut just right so that it isn't a problem. Maybe some kind of very thin molding could work. I dunno.
