Your local dealer will have a little plastic, fake chrome looking decorative cap that pushes in the hole. Five bucks or so. I believe they come stock on Heritage Springers (they're stored under the seat when riding two up, or at least so I've been told).
One of the HD seats has an extension that covers that hole up when you run the solo seat. Little creativity and you may be able to make it work with yours. You know any PJ's from Patrick? I'm a LM from Patrick.
On my O3 fatboy I used a chrome button head allen bolt and chrome washer normally used to mount the trim on cowbells for a FL front end.
Parts left over from working on a friends bike. I had to remove the special nut and replace it with a 7/16 fine thread lock nut.