here is another link for the turnkey clamps on amazon. they are about twice as expensive ($4.07 for 2) as when the OP listed them back in '09. still cheap insurance.
also, if you have a West Marine near you, they also carry 300 stainless clamps in size 20 for 1.59 each (breeze booster is the brand name). they also have 316 stainless for 5.79 each. all found on their website www.westmarine.com
btw, the company i work for, Master Lock, makes a pretty nifty tailgate lock for $20 that will fit toyotas. hope this isn't against the rules (i am not a vendor), but here's a link:
you can also see a vid by Mike Rowe demonstrating tailgate theft and security there as well. he is our new spokesman. shameless plug.