That's definitely a lot of putty, you don't need that much to secure it.
Ord Mantell shared how he assembled his based on the original, using some visor material and some glue:
I am LOVING this thread! As someone who is new to Fett building, it’s fascinating to read about the differences not only between ESB & ROTJ but from scene to scene. Continuity obviously wasn’t a huge priority for Lucasfilm! The effort you’ve gone to in perfecting the accuracy of this build is...
www.thedentedhelmet.com
Even if you don't plan to match that exactly, you just need some glue along the edge and some strips of plastic if you want to secure it even more.