I am not a cake maker and when I found out the girl who usually makes my cakes was on vacation - I decided not to buckle under pressure! I was not about to try a Darth Vader cake and R2D2 cake looked like it might be remotely possible. I had done a Google search which took me to your site and I liked the cake submitted by Bethaney T (Syracuse, NY) so that was my inspiration.
I used the Pyrex pans I had which already had round edges for the rectangle cakes. I used a 13X10 for the largest part of R2D2 and then I cut a smaller 10X7.5 in half and only used part. I then used a 10.5 circle pan and cut it in half - using half for R2's top and then I cut of the ends of the other half of the circle and then cut it in half. It was easy to make with regular size pans, but the problem was having something big enough to place it on. I used one of my larger cookie sheets covered in foil, but the edges of his "legs" hung off a bit.
I "glued" the pieces together with royal icing and then froze it so I could do a "crumb coating" later before icing it with a buttercream icing. I used a great new recipe for marshmallow fondant to make the blue detail pieces. I actually took wax paper and laid it lightly over and sketched with a sharpie to make makeshift pattern pieces which worked great! Then I did the final details with royal icing I colored gray.
One important thing to note is that if you use fondant- you cannot freeze or refrigerate it as it will get condensation on the fondant when it comes out and gets spots.