I had obviously overbuilt the cheeks/tubes. They stuck out way too much. All that was needed was "from the corners of the eyes and forward to be bulged out and then carefully blended with the rest" as a friend said.
I spent the next couple of days scrapping off all the clay that I'd applied to the cheeks and redoing them. I reused the aliuminium tubing from before but I cut the pipe in half all the way down the tube and then slightly bent each piece it to get that flowing look, slightly bowed.
By this time, the inside piece of the nose had been removed too and been replaced with clay. Id also slightly modified the whisker on the right (as you look at the mask) so that it was a closer match to the other one and was happy with the result. To my eyes, it had stopped that side of the face looking all slumped. The challenge had also been to try to keep the asymmetry (it’s amazing how you subconsciously try to make the left and right sides matchup).
I next started to plug all the gaps in the mask in preperation to casting it. The inside of the mouth had some plastic backed hardboard added and the tusk holes were filled in. I then spen the entire afternoon smoothing out the clay by brushing a thin layer of Vaseline over it and working it with the brush. Once I was happy that it was as good as I was going to get it, I set the clay by applying baby talc over the top. The clay was now feeling like freshly set plaster.
I'd also filled in some of that area in the triangle under the chin to stop any undercut in the mould.
The only area left open now was the eyes themselves: I could either make some clay ‘lenses’ or use some old sunglasses (or similar) to plug the eye holes but I didn't need to decide on that just yet. I applied some shore25 RTV silicon to the mask as a test and things look very promising. I just needed to get some thickener and some more clay for the edges before operation ‘cast a mask’ could start.