I have never seen a convincing explosion in a diorama, because the the available materials are all too solid. An explosion is a dynamic event, and consists of hot gasses and bits of flying debris (i.e. mostly nothing at all). Most attempts end up looking like giant mutant mushrooms. An explosion, even one captured on high speed film, looks like a blur, and I how do you model "blur?"
Best advice, in my opinion, is to depict the aftermath, not the explosion itself.