The stock seats have a place to keep your paper work. Even big enough to carry the owners manual. Just a plastic door that clips up on the seat.
I carry the reg and INS card for all the bikes in the inside pocket of my riding jacket. That way no matter which bike I ride I have the paperwork with me.
There are lots of stash tubes out there. The moiunt in many places on the bike.
Here's a few J&P have. Some mount any where, some mount on the lisence plate. One is a footpeg, ones a set of grips.
Leo