Here is the template I used to modify the Simpson Strong Tie A33 carpentry bracket. I printed them out, taped them on, then got a marker pen and traced the template onto the bracket.
Mounting Bracket Template by
Gregg.F, on Flickr
There is a slight vibration with the current setup but the only time I can notice it is when I am directly behind another vehicle. Friends suggested that I attach a piece of flat stock between the two mounting bolts. This would tie the two brackets together and (possibly) reduce any vibrations.