Ok, progress - As expected, once the S-Trim was installed, I had a big gap near the ear, so I cut a little triangle from ABS, glued it in:
Whipped up some ABS paste with shavings and Acetone.
In hindsight, I should have mixed it up more to make it more pasty, as it was hard to smooth out. I ended up giving up and just relying on the trusty dremel to smooth it out later
But it turned out pretty good
And the end result, I think the helmet is finished