Unfortunately objects on cloud storage are generally immutable, which means that trying to do the sort of random access read and writes that a filing system does isn't possible with one large object.
It is possible if you chunk the object up into lots of little bits, but rclone doesn't support that yet.
This will show you the default place for storing cache files
$ rclone help flags cache-dir
Usage:
rclone help flags [<regexp to match>] [flags]
Flags:
-h, --help help for flags
Global Flags:
--cache-dir string Directory rclone will use for caching. (default "/home/ncw/.cache/rclone")