I have a never-used KTM Hard Parts tail bag, with all hardware. I've had the bag for several years, but have never used it and currently the only two-wheeled demon I have is pedal power with no rear fender to mount this thing to. With just a quick Google search the cheapest I saw one of these is $65.99. It appears to be waterproof as it is a rubber-like dry bag material. Asking $40, located in Round Rock and will be happy to meet within a reasonable distance, or ship on your dime.
Here is a link to what it is:
Link
Here is a link to what it is: