I used a truck off the bottom of a roller skate. I ended up hack sawing /drill /tap a 1/4 20 hole in the truck and used the rear fender tab hole in the frame to make a good solid hard point for the rack arms to bolt, also used a couple spacers. Easy to work aluminum. I used a bigger skateboard truck in my buddys bike. Next time you get the chance, pick one up ( a skateboard) and look at it.
check out my sail album for more pics on this.