This is something that I was speculating post-election and it blew my mind that if they thought to do this ahead of time how ironclad it would be.

IR ink producing cryptographically unique 'splotches' on every ballot, or pre treated/printed paper delivered to whomever was contracted to print the ballots.

Since you don't have the private key, you can't generate new 'authentic' splotch patterns, only duplicate existing ones, but anyone with the public key can check and authenticate the splotch pattern.