I actually remember seeing that picture accompanying a review in the Guardian Weekly... and reading the review without really knowing about the play.

The box in this production was sort of like a giant plastic trombone case that stood upright. Northumberland stuffed Richard into it and latched it after the farewell-with-queen scene, and it sat on stage until the prison scene, when the jailor came on and let Richard out. (There was definitely some creepiness created in the intervening scene by knowing that Richard I In That Box the whole time). Exton turned it on its side and dumped Richard in after killing him.