Hi Greg
My cover is about 1400x1000 and I use 6mm acrylic in two panels. If you use 'corner cubes' and core screws, the acrylic can slip into the extrusion slots. You'll need a centre support - use 40x20 (mine is 20x20 and could do with being bigger).
The corner cubes protect your fingers from the sharp edges of bare extrusion.
It can be any old colour - it's really the 'welding flash' you need to protect against (as well as any wayward laser) - consistent with the user requirement of watching the fancy lights - it's a bit like watching an aquarium or a pen plotter
You might be able to see my cover across on my buildlog.
Vent slots - you're probably right
However, I just have my cover open 4mm with a 4mm gap all round -I haven't needed to cut vent slots (yet) - I tested the setup a couple of times with an orange smoke bomb and it cleared very quickly (neighbours were a bit concerned 'though
)
Cheers
Neil