One simple solution (2018, FCPX 10.4, macOS 10.13) is simply to keep render files external to your FCPX library file in a location which you exclude from Time Machine. Apple macOS Time Machine can exclude folders, but not individual files or file patterns.
For your FCPX library, examine the Library Properties. In the Inspector window by "Storage Locations" click the "modify settings" button and then change the Backups location to a folder, not the library. See FCP X help article "Manage Storage Locations"
For example you could create and choose a folder /Users/myhomefolder/Movies/FCP X Cache And Renderfiles, and exclude it from Time Machine in System Preferences -> Time Machine -> Options.
Each library will have a .fcpcache bundle of the same name generated for it. These files can be regenerated and re-rendered from source media if they are lost or deleted. See "Manage Render Files"