Cpt. Ron started this, with his free tire, so I thought I would give something away too...
I have an RKA magnetic tank bag that has served me well for many years, but I can't use it anymore because my bikes are all plastic now. It has a clear window on the top for maps, can hold a ream of 8.5 x 11 paper without stretching, and can expand upward to about ten inches high when necessary. The magnets are super strong and it has never shifted on me when in use. I've never worried about scratching the tank with it either, except the one time I set it down while taking bike photos in a San Francisco boat yard and it picked up a bunch of iron filings and stuff on the ground there. Fortunately I was able to clean it before putting it back on the bike. Leave it on the bike and you're safe.
There are no conditions attached to this other than that you need to come pick it up at my home in Folsom. Send me a PM if interested. And if Cpt. Ron wants it, he gets higher priority.
That's it on the bike with me behind it in this photo: