file-suffix.tcl
I don't like using subsite_id. How will I know which file is which when I am in emacs? Since you'll need to use a regexp anyway I don't think using the ID is any benefit.
If you have multiple subsites with the same name, they'd need to be nested so you could do
file-path-to-subsite.tcl
which is kind of verbose but much more clear what you are actually editing.