Did you make the lid completely out of gumpaste? If so, try using a piece of 1/2" foam core, cover on both side with fondant and cut a ribbon of gumpaste to put around the side, (if the makes sense)
I was going to suggest the same thing.
I did use foam core and covered it in fondant. I guess it's not so much the weight as getting it to be secured to the cake.
If it sits too far forward it coveres the stuff in the box. If it sits too far back it may slide off. How can I anchor it????
You could try to anchor with either melted white chocolate or royal icing.
I used a bamboo skewer. I stuck the skewer into the lid in the center from underneath. Then the other end of the skewer went into the cake. When you place everything else around the skewer, like the rattle and stuf, it covers the skewer comletely. You will have to cut the skewer down to the right height though.