Why not just use 1 solid piece if you choose not to use colored? Would be much easier.
1.5 inch thick plexi (24x24) -
http://www.mcmaster.com/#8560k913/=1axdzz
2 inch thick plexi (24x24) -
http://www.mcmaster.com/#8560k915/=1axela
If you want colored, go with 1.5 or 2 inch depending on if you want front and back colored and put a thin colored layer on.
Go about half way down for colored -
http://www.mcmaster.com/#cast-acrylic-sheets/=1axkqn
I haven't been able to find 1.5 or 2 inch thick colored unfortunately. I think what i would do is go with 1.5 inch thick and put a sheet of .236 colored on the front, leaving the back clear. If it has to be thicker, 2 inch thick with .1 thick colored sheet.