How thick should the Plexiglas be?

That is actually a tough question to answer. Certainly no less than 1/4".
It depends on the shape and if it is framed or unframed. Unframed, if there
is a fairly constant curve over a relatively short distance 1/4" will do.
If there is a flat area in the middle you need a frame or go up to 3/8".
The main danger is someone grabbing the top and pulling on it as they stand
up. That is a natural action and hard to prevent.

I have had good luck trimming to shape with a sharp new fine toothed
sabersaw blade. Cut a little outside the final size and sand down with 80
grit on a ROS. Move the sander quickly to prevent melting. Work up to 240
with the ROS and shift to 600 up to 1200 on a sanding block. Finish up with
a plastic polish on a small buffing wheel. A quicker alternative method
would be flame polishing but that takes a lot of practice and an artist's
touch to avoid bubbles. I would not risk ruining the project that way
unless you are really good at it.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com