Personally, I'd avoid the plexi in favor of glass due to glasses easier cleaning. I've got 2 observation hives, one with glass and the other with plexiglass. The plexi gets increasingly harder to clean with each cleaning, eventually loosing much of the transparency. Haven't used the plexi one for quite a while (always intended to replace the pled w/ glass). Glass cleans up real nice.
I've heard of folks placing 'windows' into their Lang systems, usually a small one on the side of a DEEP (no reason a medium couldn't be used though) leaving them in place until winter, window facing NORTH so the sun won't enter the hive. I suppose someone could build several so they could be used as a complete hive!
..So, it can and has been done, I see no reason why anyone shouldn't try building one. In fact you have inspired me to at least think about this as a summer project, THANKS!