I used the aftermarket risers, available on Amazon. They're good because you can insert an internal-drive hex head bolt from the top, which is good because space for a bolt is tight underneath. You can then tighten the risers with a hex driver from the top, and a wrench held on the bolt below.
Here's ours:
I like the idea of bolting a plate to the existing handlebar attachment area (don't know why I didn't do that). It would add a little weight, but would allow easier use of a variety of handlebar mounts, and lower ones, too - just drill four holes in the right place, and any $20 riser set will work.