How do you contact non-commercial profile authors?

if their Rig Manager name matches their Forum name, send them a message.

If it does not match, try searching the "Free Rigs" Forum for the name of the Rig - with any luck, they posted about their rig when it was added to Rig Exchange.

You could create a "Thank you" post, with the Rig and Authot name in the title - the author might notice it.