Share 'Rehorning a telecaster'

I got this guitar in from a customer who didn't know how the original horn fell off but he wants a new one put on. I've had the new one cut and joined for awhile but I'm having problems clamping it. I've tried a band clamp and several variations of bar clamps and jigs. I'm seriously considering a temporary screw from the neck pickup slot and the control cavity. Any thoughts?