Not cheap ($70) but there is one solution available for vehicles with a towing kit. It's called a HitchSafe - combination lock insert for a 2" hitch receiver. Besides keys, you can include currency or a credit card! Follow the link for the pictures. The following text is from the website. NO, I do not own one - my hitch receiver is only 1 1/4" sq. (Class 2)

http://the-gadgeteer.com/2006/09/01/hitchsafe_key_vault/
The HitchSafe measures approximately 5" long x 3" wide on the combination end, and it weighs one pound six ounces. The HitchSafe’s body is made of metal with a black finish. It will fit into any standard 2" receiver, and the included EVA foam inserts make the HitchSafe adjustable for pin hole variances. According to the specifications listed on the site, "the maximum hole reach from front of receiver to middle of pin hole is 2-9/16", and the maximum body depth into receiver is 4-3/8.".

The HitchSafe is the perfect gadget for people with unused or seldom used hitch receivers. Never again will a magnetic key box of mine be lost on a bumpy back country road, and never again will I have to smash a window to get inside my locked vehicle. Well, never again after I order an additional HitchSafe for myself; this one went on my 16 year old daughter’s truck. Now I have peace of mind that she will not only be able to get into her truck if she loses her key, she’ll also have access to the emergency $20 bill that I’ve tucked inside. If your or someone you care about has a vehicle with a hitch receiver, I can highly recommend this ultra-secure method of carrying extra keys and cash.