The small miniature had various model kit parts on it (there are thread on here that go into that over in the relevant section). To build the full-size setpiece, they mainly just scratch-built enlarged versions of them, from what I've seen in photos.
I'd suggest approaching this a bit like an astromech build. The main body is easy to scale and has a simple basic shape. Draft some plans, taking inspiration from one of the common R2-building methods -- lightweight wood framework, with plastic skinning over the outside, with cutouts for the various large details ("vents", legs, blaster, neck...). Enlist the aid of a local reprographics company to do the large printouts for those plans/templates.
The Weber grill shoudl work as an "inner shell" for the head. I was planning to use a couple deep-dish toboggans for mine, built up a la a forensic-anthropology skull reconstruction -- "plugs" to indicate depth. I was then planning to model the head in its entirety (minus fine details), and do it as fiberglassed pepakura anchored to those depth plugs. Lenses and their bezels I was going to 3D print, along with as many of the large and small detail pieces as I could (unless I surprise myself and find actual glass or plastic items to work for the lenses without compromising accuracy).
An additional thought to plant in your head... I was planning ot build a magnetic-levitation alcove for mine. Since building a full-scale probe droid is an exercise in silliness, I figured why not go all out.
--Jonah