Nevada72 had a great idea here and I decided to build my own set of panniers following his lead. You can too. Nothing here is particularly difficult and the end result seems to be extremely durable.
I've been using Yamaha's 39L top case and a 40L dry bag strapped to the passenger seat. That works fine, but the dry bag isn't an option when riding with a passenger.
The number of times I'd actually need panniers is close to zero, so spending $900 on a factory bags wasn't likely to happen. With the cost dropping to $130 though, that equation changes.
These bags add 29L of storage capacity per side and appear to be completely water and dust proof. I've sealed the screws that penetrate the bags with Yamahabond to keep them both air and water tight.
I also cut down the bags exterior webbing flush with the rest of the bag where the mounting brackets attach (using a wood chisel). This provides a flat surface for the brackets to be bolted to.
Rather than use JB Weld to make the tapered wedges that fit into the stock luggage bracket I used West System epoxy mixed with strands of carbon fiber I had lying around. The aluminum angle it's cured to was drilled to allow the mix to harden through and around the aluminum angle. It's bulletproof. If I'd do it again through I'd use epoxy putty that's kneeded together to mix and cure. This would be less messy and easier to form. Saran Wrap allows mixture to both form to, and harden, in the luggage bracket and allow it's removal when cured.
So for about 8 hours of my time and $130 I've now got another option to haul luggage around should the need arise. With snow on the roads, I just need to wait until spring to try them out.