So there three faces, then a removable door so that the top vivarium can be cleaned out. I used plastic cornering to slot the clear plastic door into place, then drilled a hole through both materials and into the side wall so that I could slot an Allen key in, in order to lock the enclosure.
There’s also a wooden handle to prevent the plastic door from flexing, because my snakes are quite strong.
There two vivariums are connected by a hole in the top of the original vivarium, drilled using a tank cutter, and the top vivarium is secured using screws.
Currently I have only seen Elias slithering about up there, but he seemed quite content up there.