I think I still have a tank bag from the 70s had a nice clear cover for maps on top. Trouble with the valk is the raised gas cap is in the way (but can probably still work). My tank bib has a nice ladies clutch purse sewn in and the fold down flap has a clear plastic cover on the inside. Reach down, lift the cover and look at my directions. Only problem is, only a 3 X 5 card will fit in it. So I don't use maps, I put large print turn by turn directions prepared before departure (like Rob). If I get lost, I have to get off and consult a map in the saddlebags.

Here's a magnetic map case, with no bag. (Maybe the one referred to above)
http://www.motorcycle-superstore.com/3393/i/river-road-magnetic-map-pocket
I've also seen guys use a grease pencil with turn by turns on the inside of the shield, or turn by turns on paper.... blue painter's taped to the inside of the shield.
I've heard about these new-fangled GPS thingies, but some old dogs can't do new tricks.