Hiding a spare key under a rock is a great idea. Until someone comes along and moves the rock, finding the key underneath. This instructable will show you how to create your own fake rock that will securely hold your spare key.
Step 1: Materials
House key (you'll want to use your actual house key)
Several disposable cups
Oil spray or other release agent
Some plastic cling wrap
At least one latex type glove
A paint brush (I used a foam brush but it doesn't matter what type you use)
A stirring stick of some sort
Cheng Shapecrete Concrete
Step 2: Mix Concrete and Form Your Rock
Add water to some of your concrete depending on how big you want your rock to be. Mix it fairly stiff, as it will want to slump as it hardens up.
Place your house key on a piece of cling wrap, and spray both with a generous amount of cooking spray. You don't want your key to be stuck in concrete forever.
Now comes the tricky part. Take your mixed concrete, form it into a lump, and put it on top of the key. Be sure to smoosh it into the key well. You need a very good impression. Make it look rock like. Give it a lump or two, maybe a dent. Make it a bit higher as it will fall a bit before it dries. When you think the shape is about right place some plastic wrap on top of the rock and press gently. This smooths the texture a bit, giving it a more realistic look, in my opinion.
Step 3: Fish Out Your Key and Seal It Up
After letting it dry about a day take a look at your rock. Your key will probably be partially covered in concrete. Pry your key out. Clean up any pieces of concrete that may be hanging around the impression area. The key will now stick nicely when placed back into its impression, as shown in the third photo in this step.
To seal it I used a stone tile sealer that listed concrete pavers on its list of items to seal. I used a semi-gloss. A high gloss would not look natural to me. Use your brush to paint it on according to the directions on the bottle. Use free labor if available.
Step 4: Put Your Rock on the Ground
Place your finished rock on the ground somewhere handy. If your rock has sharp edges you made need to pile some dirt or such around the base to help the illusion.