Well, after I finished my grading I got to work on this. It is now 4 AM but I am done! It probably isn't the greatest secret in the world, but it is what it is at the last minute.
Oh, and the pdf goes along with it provides some backstory details (and some intertextual fun for me). You should not need it to figure out the secret (that is what all of the writing on the map is for) although the last page has the solution, printed upside down of course as with the young sleuths of my youth
There are two maps that go with this, the original I posted along with the detail map included here (which is just a blow-up of the vector original, no tweaking other than re-positioning names and such). I have also included a zip file with the original tif because the gif screwed up my colors (as usual), but the tif when uncompressed is something like 5 meg which will kill those folks on dialup.