SQL Plus does not appear to be bothered by the direction of slashes, ie “/” or “\” in its scripts
Hence as long as you have a “C:\tmp” folder on your Windows drive (or wherever your SQL Pus binaary is installed/running from), you can do:
spool /tmp/blah @/tmp/my_file.sql
and it will work on Unix and Windows without alteration.
(This also works for SQLcl as well by the way)