Hello cake peeps!

I've got a Minecraft mosaic 8" cube to do for this weekend.  In the past, I've placed all of my mosaic tiles directly on the buttercream and it's never come out quite as smooth and straight as I would like.  Has anyone placed all their mosaic tiles on a fondant backing and then just applied the big fondant square directly to the buttercream?  I'm worried that a panel that large would collapse under its own weight... Please share your thoughts and experiences.  TIA!