Thanks everyone. I’m glad my friend thought about me when she saw it; the best part about it is that it was only $5.
The top of the cube is open. I plan on making a front opening tank out of it; but first I am going to go to the local pet store to get some aquarium sealant so I know its water tight.
Plexi is a type of acrylic, like Lucite, Crystallite, ... Depending on the type of silicone it may or may not bond verywell, but it will not dissolve the plastic. Why don't you do a leak test to see if you even need the silicone?
The best thing to do IMHO is to "weld" the plastic back together with this stuff: http://www.tapplastics.com/shop/product.php?pid=130&. It will look great and the plastic weld actually joins the two pieces to form one large container. Its pretty cool. Good luck and my offer still stands on throwing it out for ya