You can get a President's Special router plate from Grizzly for $13. Lots of members, including me, have made their own from sheet plastic. Some have just screwed the router up under a sheet of ply. There is no set rules here, what works is the bottom line. Most plates are between 5/16" and 3/8" but quite often the actual measure is metric because of the origin.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.