A friend of mine has a Topeak rack that appears to have built in spacers. It might be this one.
http://www.rei.com/product/735847/to...th-disc-mounts
A rack with the spacers built in is designed for that amount of strength in a cantilever design. Simply putting an inch long spacer on a long bolt would mean that you are depending on the strength of the M5 bolt which could bend.
But I so much wanted to run off on a tangent ... ...