Ok here are the pics I promised.
I think I said earlier that I had the Pelican 1500, that was NOT correct, it is really the the 1520
. It is mounted directly to the top of the Touratech plate with a thin slice of rubber in between to keep things from chaffing. It just clears the top surface of the passenger section of the seat by an inch or so ...
I have small "strap loops" mounted to the top and front sides. I like to do things the hard way, so mounted the strap loops on the curved surfaces of the case, so I could not use regular fender washers to back up the mounting bolts. I had a scrap of hard copper tubing that matched the radius of the inside of the Pelican and quartered the pipe the long way on the band saw and used that to back up the bolts.
To prevent leaks I used those metal washers with the rubber on the bottom (used for metal roofing) under the strap loops.
If I ever get time, I'd like to try to key the case to use the ignition key, in the mean time I have a Seasamee lock that I can open in the dark by "feel".