I had thought of that 'push down' idea myself, but after chatting with a few people who have done some work in Sintra, I was told that it probably wouldn't come out very well... which is why I gave up on a Sintra helmet project until I saw that metal one in-progress.
My understanding is that, while pretty easily bendable at lower heat levels, Sintra didn't stretch quite as easily at low temperatures. Without a vacuum forming process, once the plastic was melted to that level of stretchiness, it probably wouldn't settle smoothly into shape, leaving me with a lumpy and uneven dome.
It would be nice to come up with a method that required fewer joints on the top, but I'm not sure how. Another thought I had, other than the 'pizza slice' dome, was to make a series of four football shaped arcs that could be fit together with simple bending, and cover front to back. I haven't modelled it to see if it would work though. Actually, I haven't even bought the Sintra yet...I'm still working pretty much a priori here.
