Yeah, unfortunately, Viewport Frames are extremely limited in terms of lighting and resolution.
Non Viewport:
Viewport:
Source: ViewportFrame rendering shouldn't have a resolution limit & should not have a white outline
Alternatively, couldn’t you just fix the player’s camera on a replica of the box with the desired backdrop that you want?
I could if it was a staged backdrop. Instead, I want to put just the box in view in front of the skybox.