Hello,
yes, it is the C3PO from UHU,. The R2-D2 is from the following link:
http://www7a.biglobe.ne.jp/~sf-papercraft/Gallery/r2d2/r2d2.html
The R2-D2 modell is nearly the same size, so you don´t have to rezize it.
Here are some pictures from assembling the diorama. The background is...