Flat black laminate is easy to come by, and curved on the insiede of a slot port can do two things. Eliminate black pain which can leave an unprofessional look, and can perfectly round the corner. Alot of people use 45's in the corners to help airflow. That would be the same concept, just smoother. Could put a 45 behind it to back it up and reinforce it. I would probably pour some resin/bondo mix behind it to fill in the voids so that it wouldnt vibrate at all. Just a random thought after all of this.
The steps for the lay-up of your vaneer should go more like, spray (or brush) contact cement... stick it, flush trim it, and sand the corners ever so slightly. Vaneer is easy to burn through. Not only that, but if your vaneer doesnt have a paper backing, you will want to use wood glue, other wise the contact cement will bleed throughb the fibers of the wood. Just make sure your box is perfectly flush trimmed and sanded so you don't see any imperfections, or so that a tiny blob of glue or somthing doesnt crack the vaneer. Just a bit more from my random tid bit of thinking from reading all of this..
