Use two 7/16s bolts x like 2.5" (don't quote me on the size though) with washers, put them through one of the chain links and thread them into two head bolt holes directly across from each other, but not completely tight so the chain dosn't scratch the deck surface. This is what i did and it worked flawlessly.

 

Almost every tapped hole you'll find on the outside of the block is 3/8" by 16 thread count. I'd go to Home Depot or Lowe's and grab a handful of assorted length bolts in that size and some washers as well.