I would have thought that 6 plexiglass squares, glued together in the correct orientation would form a plexiglass cube
do a quick search on google, there will be loadsa 'plexiglass basics' pages, and lots of 'how to use plexiglass', 'forming plexiglass' you get the idea!
good luck, it really is trixky stuff to get perfect, it's VERY easy to do a botch job,
the hard part is getting the measurements just right :-)
you may find it easier to make two plexu U shapes that can mate together to form a box