The ebuild for smplayer can easily be made prefix aware by prepending /usr with EPREFIX. See attached diff Reproducible: Always
Created attachment 375002 [details, diff] patch against smplayer-14.3.0.ebuild
Please post emerge --info so we know what KEYWORD to add.
Created attachment 375078 [details] emerge --info As requested, the corresponding output of $ emerge --info
Done, thanks!