joining plexi glass for a display, what should I use?

I am making a 24" x 24" window, its about 1/2" thick. I am putting a
small green air diffuser (i think thats the name, just a little green
tube that makes bubbles) on the bottom and covering it with a little
sand. It will be a window that bubbles basicly.
I have some precut 1/4" acrylic plexi glass, 2 big pieces and 4 small
pieces for the top which has a small hole for the tube, bottom and
sides. What would be the best way to join all the pieces and make
sure the diffuser stays on the bottom?