So, R2D2:s head is a very specific "egg-shaped" aluminium dome, 300mm in diameter, and the process of making one is just not one that you want to do in your own garage. Although you could 3D print a dome, in parts and then assemble. We decided that we wanted closer to the original aluminium dome, that is manufactured by one of the members of the Astromech forum.
Derren Muller sells these, but only to Astromech members, since it is part of the R2D2 builders club.
At 490$ that would be considered a bargain! Its just perfect!
There is a lot of work that goes into making this dome, into a replica of R2D2, but still, its the best possible start.
On the internet you can see hobby projects going into using for example IKEA kitchen lamps, or their saladbowls for a similar look, but unfortunately the shape is somewhat wrong, since R2D2 is more "egg-shaped" and also the measurement is off, at IKEA with a 450 mm dome, you would get all measurements on your R2D2 unit wrong aswell...