Sorry, I didn't mean to post without a brief explanation.  I accidentally found something that might interest you if you live in a wet climate.  This is a dry bag from Wolfman motorcycle luggage.  They make high quality soft luggage.  I have a couple pieces for my other thumper - KLR.  Anyway, this fits quite well on the back of my Corbin seat up against the sissy bar.  Rest assured, your items will stay dry and it also works provides a little back support.  I'm pretty pleased.  Just a thought that has enough merit to post.  Best regards.