I didn't fit one to an OEM top box, but here's how I attached one to a knockoff Pelican case:
The backrest pad is an ATV seat pad from WalMart, folded in half with a piece of outdoor seat cushion foam stuffed inside, with the edges of the folded backrest sewn together. On each side of the pad I sewed a quick release buckle, with some adjustable webbing that terminates in a snap hook. I attached a couple D-rings on each side of the case. The snap hook clicks onto the D-ring, and then you can tighten the whole thing down to the top case by tightening up the quick release straps. You can't see it in the photo, but there is also a strip of two sided Velcro tape on each lower side of the pad; these go through a couple slots in my luggage rack, and keep the seat pad from shifting upwards when the passenger is getting off.
I made it this way so it could easily be removed for washing (and so the top case could be washed). You wouldn't need to mount D-rings on an OEM case; I had to do it that way because I couldn't wrap the attachment strap all the way around the top case (it would have interfered with trying to open the case). You wouldn't have that problem with the OEM case, since it's a top loader, and you could just wrap the strap from one side of the backrest all the way around the rear of the case to the other side of the backrest. That way, you wouldn't have to drill any holes in the case and compromise its water resistance. It's been a very secure mounting system, and it's been on the bike for at least 80,000 miles.