I take the glass and paper out and fill it with a piece of foam board. There i mark the dimensions on which I can place the scenario. With a air drying powder I created some small basics so that I can use them for different dioramas. Below is one with a wall.
The frame which I get for just 1 Dollar.
You have only to consider that most air drying stuff is shrinking a lot. Therefore I had to extend everything later again. My mistake!
The finished mini diorama, a German Hetzer somewhere in the west waiting for "friends"