I have similarly thought of a design for a small observation TBH that would be maybe 18 inches long and one of the two sides would be a piece of plexiglass entirely.
I figured on running a 1/8 groove about 3/8 to 1/4" inside the edge of the sides of the bottom and end pieces, allowing the 1/8" plexiglass to slide down snugly. This leaves the side at it's original specs and I might attach a board the same size as the opposite side using hinges at the bottom and a couple clasps on the ends to keep it tight. This (I think) would keep it nice and dark most of the time, and the airspace between plexiglass and wood would be a decent insulated area. The top would be tight fitting over each side and also clasp down to the ends, leaving the only opening the entrance to the tube or conduit via a window, etc... This could be covered with a piece of screen etc.. when one takes it to a demonstration or something.
I will be making one like this over this winter. perhaps I'll post a picture of it and see what you think.