For my road bike, I needed to add +/- 1cm in height for my aerobars. I have a 31,8mm handlebars and Pro Missile Aerobars.
I chose 9mm because M5x25mm is OK.
Perfect fit for me!
Simple design, but ok for me.
Printed in blue, not to fit color of my bike, but because all I had was in old PETG blue filament :)

I printed it on the side (file as is) to limit support and to increase vertical strenght. 

Elevated infill (60%) to sustain 6nm tork I have on all screws.