i bought a mopar acessory one off of craigslist. Doesn't have a stupid cinch strap style so you know your bikes are safe. it has a metal plate that sandwiches the cross bar with some foam padding.
It also has a slot going through it, so when I get home I can make a lock for it (doens't make a difference if you don't have a hitch lock though!)
This is a good picture of what they look like, but they can be found for much cheaper than that. (mine was $40 from CL)