I am actually working on my scout helmet, but a lot of people have been curious about how to go about replacing the lens with dark plexiglass. You can NOT use Lexan with this method.
Making a Fett visor is very similar to these Scout Visor pictures, but you need to curve the middle part over a wine bottle or something first to be sure it curves nice and straight in the middle. Or in the case of Jango, don't do that.
This is a rear-mounting lens.
(Scout Helmet)
I cut out the inner rim of the lens area with a craft knife. I traced around the lens that I cut out from the scout helmet(leaving about 1/4" extra room around the edge). I cut it out from a flat piece of smoke colored plexiglass (scraps are as cheap as $1.50). (Fett people-just cut out the visor and trace that)
I laid the cut out flat plexiglass lens piece onto a Lexan face shield. (Side note: I have NEVER seen a tinted face shield despite looking pretty hard).
Wave the heat gun over the plastic, working from the center out, until it bends by itself to the same shape as the visor. If you wish the curve the lens more than the Lexan visor normally curves, simply tape the two sides closer along the back. (Fett people should have pre-bent the just the very middle over a large glass bottle)
Here's a pic:
Notice I did this on a heat resistant surface...
A pic of the face shield off and the newly curved lens laying inside of it.
If you work quickly and lick your fingers so they don't get burned, you can conform the lens a bit more to your helmet.
Use hot glue all around the top and sides of the visor. The opaque kind works best for me. If there are any parts of the visor that are just slightly not close enough to the lens, the heat of the hot glue will soften the helmet right there and you can push it closer to the lens from the front. With Fett, I found it very important to watch the front and keep it pressed down so that glue did not seep under and cause the helmet to lift away from the visor. I glued one side at a time with the Fett helmet.
If you get glue anywhere you don't want it, use Goo-off to remove it, then windex to remove the Goo-off residue.
The finished helmet:
Hope that is helpful to some people.