If you want a cheap and easy way to rivet the holder to the helm, it's just drilling 2 holes then using "speed rivets", which are small rivets that you attach with a little rivet gun. You can pick up the gun & some misc starter rivets for around $15.
I haven't really seen much of any plumes pre 15th-ish century, I think the torse and mantle is more appropriate to the earlier periods, if that's of any concern.