I suggest making each employee come to a location and get a physical copy of the SPD. I would also have postcards printed up and affixed with some sort of hologram (so you know the cards are authentic) with the employees name, SSN and other employer related data (division, pay grade, location, EE-id, whatever).
When the SPD is handed off, the employee must show government issued photo identification (driver's license, passport, etc.) and a secondary piece of id. Also, as proof of receipt the employee must sign the postcard and give two thumb prints--one right, one left. Get the cards notarized, too.
Having the whole process under video surveillance is an additional safeguard. Make several copies, and store them on a variety of media (flash drives, dvd's, digital video tape, on the company's HR servers and any backup servers, copies to attorneys, too.)