Map "codeing" help needed

Hello everyone.

I am looking for some help in map "codeing". I don't have an actual map to decode, but I do have a "cache" that I want to bury and have found in say 100 years or so. This is not a "geocache" or something I want found over and over again. What I want is to make an actual map that uses real "codeing" techniques that I could hand down to my grand children or their children. Kind of a starting point for a "treasure" legend.

To make a long story short, I have access to a piece of land that has woods and a creek and a bluff and a small grotto. In this grotto there was a "bunker" for lack of a better term. A friend (the property owner) and I came up with this idea a few years ago. We shored up this room with cinder block and rebuilt a door into it. After backfilling more around the opening and planting some weeds and other foliage from the surrounding area, you can hardly see it or distinguish it from the hillside.

We planned on placing things of more historical value than monetary value into it. A couple of years worth of Time, Newsweek, National Geographic, People magazines and others all separated by year and sealed in "Seal-a-meal" bags. The kind you suck all the air out of. We are hoping this will help preserve them. There will be other things also, but we don't want to let out too much for fear someone will find it and ruin it. We are putting in some things that are very old now and have some value. Plus some family history on our families and many many pictures, again sealed.

So much for making this a short story. Can anyone here can point me in the right direction to make an actual honest map using real "codeing" techniques? Like I said, we want this to be found some day. But we want to make the lucky person who does find it earn it with research and brain work.

Sincerely,
Hat_man