I am making a 3d r2d2 cake for my sons b day next month but he don't like fondant does anyone have any tips on how to do this and get the cake smooth using Buttercream I also need filling ideas for an 8 year olds vanilla cake

I just made one earlier this month.
I got my buttercream as smooth as I could to start with, then let it crust a little, then I used a Viva brand paper towel (no texture) and a fondant smoother to finish smoothing it. I was getting tired when I piped the lines so they weren't as straight as I wanted them to be.
The legs were made from rice krispie treats a few days ahead, I frosted one side then froze them so I could frost the other side without damaging the side I just frosted.

That is awesome it is just what I needed.

Do you have to put anything on the papertowel before you put it on the cake