Hmm, I am of a different opinion. A stock 41mm uses slugs. They are short and called fork tube plugs. Making longer versions of those gave rise to the slug as cheap way to extend forks. A two inch slug is fully safe in my opinion. The bad ones are 3" and longer. The killers are the ones that go below the triple tree.
As far as I know there are no extended fork plugs available for the FXR front ends so a machinist would have to make them. Which in the end probably would cost as much as new tubes. And yes Forking by Frank makes excellent fork tubes.
Your friendly neighborhood W&W Cycle Germany, Paughco and V-Twin dealer