Plexiglass is a
superb material for speaker cabinets.
Here's what I previously wrote on the subject of cabinet material density
Polymethyl Methacrylate (PMMA) (brand names Acrylite, Acrylplast, Altuglas, Lucite, Perspex, Plazcryl, Plexiglass, etc.) and has a density of 1,190 Kg/M3
MDF, varying according to composition (might be sawdust, straw, sugar cane residue, etc.), has a density ranging from 600 to 800KG/M3
Plywood (0.75") ranges in density from 450kg/M3 to 650kg/M3
Concrete Block is 1,281 kg/m3
So acrylic is almost as good as concrete block.
Beautiful AND functional, what's not to like? Oh, right, the cost! Three quarter inch thick acrylic is about $20 per square foot, and one inch thick runs about $30/sq ft. So the materials to fabricate one of these cabinets would cost about $400- $600 depending on thickness.
Acrylic is brittle and readily scratches, true, but this can be fixed with some plastic polish and a sanding block. Maybe some progressive 800+ grid for the bad scratches.