There are many ways to attach the radio mounts to the aircraft. One solution that I am using is to attach T-channel to the aft side of the instrument panel. By sliding the attaching nut into the channel I can attach the radio mount and slide it into up or down into any position necessary. The images are not clear but you can see the t-channel clamped to the instrument panel.
I need to drill the the attaching locations. The channels are clamped into the drill press ready for drilling.