Plaster of Paris would be rather brittle for your project, and the shells and buttons, etc, might break off quite easily. Any flexing of the boxes might damage the plaster, too. This plaster is known for its quick-setting property, so that's quite normal. Like Naiomicorlett, I'd suggest Polyfilla instead. It has resins that stiffen the plaster and make it harder but much less brittle. It sets more slowly, too, allowing you about 30 - 40 minutes to work with it. Being slower than Plaster of Paris, it might take 24 hours or more before it dries out fully.