Set a base directory for a local file system?

I feel there must be a simple way to do this, but maybe I’m just being dumb. What I’d like to be able to do is run rclone sync local: dropbox: to sync my “remote” named local to my remote named dropbox, where local is a directory on the local drive. That is, this command should mimic rclone sync ~/Dropbox dropbox:. But local just points to my present directory, rather than a directory I have chosen. Is there an option to use a local remote the way I want?

rclone sync local:/full/path/to/dir dropbox:

root@cloudc:~# cat .rclone.conf
type = local
nounc =

You can also make an alias remote if you want.