Dittos to everything Tom said!
I'll add two more caveats about showing dioramas:
If you use or need electricity for your diorama, check first with contest/show organizers. Some venues must make special arrangements or provisions for electric power, or locations with power are limited. Some venues don't even have provisions for electrical power at display tables. I've known of venues that charged a fee for any plug-in, and the contest/show organizers will generally have rules or guides that apply to requests for electricity.
Also, if your diorama is populated only with die casts, check first with the contest/show organizers for applicable rules. Some events allow only plastic models and prohibit any die casts. Some events allow modified die casts but not unmodified ("store-bought") die casts. Some events will allow 'display-only' status for dioramas populated with only die casts, but won't allow them to be entered into competition.
It's best to know the rules beforehand, as Tom said, rather than show up and find your diorama can't compete or be displayed. Yelling and screaming because you didn't check the rules first generally won't change the rules.