Panels page variant that targets a single node
Assuming you already have the node override Panel enabled:


Edit the panel page for the node override (node_view for node/%node)
Make a new variant, enable selection rules
For the selection rule, choose String: Url Path
In the criteria, keep Allow access on the following pages selected
In the textarea, add the internal node paths for this nid(s) you want.  To target nid 1234, use

node/1234
Configure the test of the panel as needed.


Just make sure this comes before any less-specific rules, like matches on content type.