It's not Plexiglas, it's probably acrylic. Plexiglas is a safety acrylic and it weighs a ton. Just ordinary acrylic weighs a lot too, but a PC case made from genuine Plexiglas would be seriously heavy.
Plexiglas is a tradename of a big German chemical firm (I used to work for them) and they don't sell to any computer case manufacturers. I had the only Plexiglas glazed case about 3 years ago and it was made for me as a design project by a Degussa/Roehm apprentice and was a standard mATX computer case with a sliding 'sunroof' that opened when the PC was switched on and a fan rose up to cool the PC. It was a work of art and I gave it back to him when he finished his apprenticeship.
Acrylic cases are a pain. Don't do it. You need to handle it with gloves to avoid getting fingerprints on it. The screws are threaded into inserts in the acrylic sheets that generally start to turn once they've been screwed up tight once or twice. They generally don't line up square and all the ones I've seen (Akasa, Jellybean, ClearPC, Sunbeam, Mountain Mods) just look like poo with a computer inside them. And then there's the dust build-up issues. Because you can see everything, and the acrylic is slightly electrostatically charged it actually attracts dust.