How can I link to the most recent report on a given file or URL?

You can use the following links:

https://www.virustotal.com/gui/file/<resource>/detection
https://www.virustotal.com/gui/url/<url's SHA256>/detection

Where resource is one of:

The MD5 of a given file that was scanned by VirusTotal.
The SHA1 of a given file that was scanned by VirusTotal.
The SHA256 of a given file that was scanned by VirusTotal.