"Forced Perspective", which is crucial in most "in-flight" shadow-boxed dioramas, is done easily by mixing scales... Here's an example (although it's not a shadow-box), with models built and photographed Heinz Johannsen of Rendsburg, Germany. The Mustang is 1/24th and the Me 262 is 1/48, allowing the models to be only a few inches apart, yet giving them a realistic scale distance..
Also notice that the models themselves are "off-balance", thus giving the viewer a better sense of "action" taking place, since neither one can stay that way without soon having to change diredtion in order to either escape or maintain contact...
This's "Old School" model photography (What else ya expect from me?), with no PS involved...The models are suspended vertically over a horizontal backdrop and held in place with stands that have alligator clips on the end to allow them to be positioned "just so", and lit to match the lighting of the backdrop...
The camera is on a stand as well, looking "down" on the diorama, as this allows the stands to hidden from the camera by the models themselves...
Here's a (very) rough sketch of the set-up..
My current shadow-box project is the scene from The Blue Max , in which Stachel's Pfalz D III is shot down by SE-5s after he saves MvRs life... I'm using the Monogram 1/72 Fokker Dr 1 from the "Red Baron" car kit in the back, the Eduard 1/48th Pfalz D-III at center, and the 1/28th figures from the Revell kits of the Dr 1 to create "Stachel", who is standing on the ground, wounded in the arm, away from his burning Pfalz, watching the Baron waggle his wings as he flies away, and mouthing the words, "You son of a B*tch..", in the foreground...