1. You can get glass shelves for Besta cabinets. INREDA glass shelves.
2. You absolutely cannot fit two rows (one above the other) of 1/6 figures in a Besta. Once you add a shelf and subtract the thickness of the shelf (even a glass one), you will have much less than 12" per shelf. The best you can do is 12.5" for one shelf and around 10" for the other. So you'll need to keep one shelf exclusively for hobbits or child figures.
3. An offset (half depth) shelf won't work in the Besta extension units with 1/6 figures - there just isn't the necessary depth.
Custom build something out of MDF - it will be super quick and super cheap. Sand all the edges and seal them well before you paint, then it will look seamless. You can easily add sliding glass doors that you can have custom cut by attaching plastic channels which can also be bought cheaply online.
