Glad to have the secret of this glitch finally exposed. There were a lot of items in that one box, but not everything. The possibilities really are infinite now.
the easiest way to get into the rooms- challenge maker. an they interlock so...I assume locators with a toy box door also but then you need a way out. The challenge maker at the moment best option. Collectibles cannot be placed inside but locators can so you can spawn enemies etc.
I can't.. how are you doing it ? I placed a Command Center and a Garden Room in the toybox...I can get inside but I cant put checkpoints,teleporters or cakes/orbs