Acrylic or Lexan would work. The thickness is going to be determined by the amount of water to hold. You can seal it with a good silicone from Home depot.
I would not trust the intregidy of regular plexyglass I have worked with it restoring my Vette and went to bullet proof Lexan. I would imagine Acrylic is more cost efficient though, Lexan is pricey. Plexy is brittle when cutting and drilling.
I do alot of acrylic work, so I thought I would chime in.
Acrylic and lexan are not the same thing. Lexan is polycarbonate, and is not as easy to work with, and the appropriate adhesives are not readily available to the weekend warrior.
Assembling acrylic with silicone is a bad idea for all but the smallest vessels. Silicone does not adhere as well to acrylic as it does to glass, especially on an unpolished edge.
You could use an appropriate acrylics cement that is available through a plastics supply place.
