Proper syntax for plain-file with sudoers-file

[白い熊＠相撲道]

I've been using
(sudoers "xxx...")
in my config.scm file a long time. 

Now, for some time it's been warning me that using a string for file “sudoers” 
is deprecated and to use “plain-file”. 

So I've tried
(sudoers-file (plain-file "/etc/sudoers" "xxx..."))
but I'm told: “invalid character `/' in name `/etc/sudoers'”

What's the proper syntax? 
白い熊＠相撲道