I would avoid using felt or anything like that. My dad built me an oak display case for a couple rifles and lined the barrel hooks with felt. I don't know if it was the type of glue or just moisture wrapped in the felt, but the barrel on a shotgun started to rust where it was touching the felt.
It cleaned up ok, and after treating the felt with some gunstock wax its been fine ever since but it would have been bad if it had gone un-noticed long term.
Fix it right the first time...
Use Baling Wire !