Trimmed the chin cup.
Before:

After:

And added the cut-outs for the elastic strap (temporary elastic shown in picture):

Continued work on the helmet.
Drilled holes and test fit the back greeblie:

Test with the trim:

Then worked on placement of the rangefinder, disk, and screw cap (trim is there just for visual placement of greeblies):

I'm going to attach the rangefinder before painting (rangefinder is olive, but antenna remains metal), but I have a couple of decisions to make:
Q1. How to attach the hollow-back rangefinder? Use screws from behind or just glue the edges?
A1. To ensure it's secure, I think I'm going to do a combination of both.
Q2. Do I attach the metal antenna before or after painting?
A2. I don't think it will be feasible to paint the rangefinder while the antenna is attached, so I was thinking of cutting a little access hole, on the helmet dome, behind the rangefinder. This will allow me access to the screw to attach the antenna.