Being a cheap SOB I refused to pay $100+ for a top case mounting bracket, so I cut a piece of 3/4" plywood to fit the mounting bracket that comes with the 23" X 17" Chinese top case that cost me $54 on ebay, including shipping, and for which Saddlemen charges $150 after putting their logo on it.
I removed the two 8mm Allen head bolts that hold down the rear of the stock rear rack, bought two longer ones, then drilled and counter sunk two holes in the plywood the precise distance apart as the two mounting holes on the rear rack. I also counter sunk and drilled one more bolt hole in the plywood at the rear of the wood and through the rack for stability. I belt sanded the edges of the plywood, sprayed it flat black, then bolted it to the bike.
I fastened the top case mounting bracket to the plywood with four bolts, being able to reach underneath the plywood to fasten the washers and nuts, otherwise I could have used wood screws into the plywood.
I now have a top case that cost me less than $60 total and will take any strength test you could devise, hell, the case will break before the mount because virtually all the stress is on the bolts and plywood, not on the bike's rear rack.
If I had a camera I'd post pix but I'm just learning my way around this here cumpooter thingy. My advice: don't let that warning on the bike's rear rack scare you from making your own mount.