That look has been done before; remember the feet on your grade school chair/desk? Biter!
If you're not using the bars that used those risers, do you just cut a random set in half? Is that strong enough to hold half a bar with a few inches of grip? School me.
I myself would mount them, twist the bars I inserted back and forth a bit until I was happy with the positioning, then weld those things right to the risers. Viola, no clamp strength issues.