Glad to see you came across this topic trooper.
As I've never actually felt the material of these things yet, What would you say about heat?
My technique would take the pepped visor, Bondo it etc etc. Then afterwards place the sun visor over the top of the
visor and used a heatgun or a blowdryer, So it would melt, and under its own weight form to the same shape of the visor, Would this make sense? Besides your cutting technique its the only other idea I've come up with.