I did the Odier Bike handlebar extender, Amazon, $16 for my Radcity Step-thru. That exact model is unavailable right now on Amazon; but, similar models should perform about the same. I don't have an issue with the extender slipping with the extra LED light, Topeak Aero Wedge pack, and cell phone attachment (Night Rider Pro 1800 battery inside the Topeak bag). I still have enough play to move the arm up/down if needed.
The space under my LCD display on my Radrover is wider to mount a extender under the display compared to my Radcity. I would look for an extender you can adjusts the width of the arms to fit inside or outside of the LCD display. Some extender have fixed width arms and that type wouldn't fit on my Radcity.