Finish a symmetrical map
Canvas, 5 4 bytes
║Ｑ↷↷

First Canvas answer, so let's start with an easy one. :)
-1 byte thanks to @dzaima.
The slashes are automatically converted when mirroring or rotating in Canvas.
Could have been 1 byte ╬ (Try it online), but unfortunately it also transforms the dots . to single quotes ' when mirroring horizontally.
Try it online.
Explanation:

         # (Take the multi-line input implicitly as canvas object)
║        # Palindromize the canvas object (without overlap)
 Ｑ      # Output it with a trailing newline (without popping)
  ↷↷    # Rotated the canvas object that's still on the stack by 90 degrees twice
         # (and output it implicitly as well at the end)