Alright time for a big, big update! I am now about 95% of the way done! I have key slots (thanks to the sacrifice of a broken fan!) and cheeks!
First the fan, It's been sitting in the garage and collecting dust, I brought it out so I could use it for greeblies and I was totally stuck on the key slots and getting it right but not having huge holes in the back of my helmet when inspiration struck.




I was able to trace inside each rectangle and cut out the key holes individually allowing me to keep some strength and structure to the back of the bucket.
Then came the hard part! The cheek and face plates. I apologize for the lack of photographic evidence but take my word, I spent 8 hours over two days prototyping and fitting and bending and redoing the cheeks. I went through I think two card board boxes (my practice and templates) and a 1 x 2 ft piece of sintra trying to get the fit right. Now it's not perfect by any means and I still have a lot of sanding and filling (and sanding and filling) in my future to get these things to look good and then some sanding and filling before I'm done. But this is still big progress and here are the photos.



Again there were moments of inspiration here. I used the leftover cheek cut outs from the bucket for the inner plate by sheer accident. I was having so much trouble getting the bend right and by chance I just picked it up and checked the fit and boom, it was perfect!