According to the picture, It doesn't look like the DF QR hub accommodates for the horn wiring through the QR itself. Switching to an NRG QR hub may help make it a lot easier and cleaner. From there, the only complication would be figuring out how to provide a switchable circuit to the horn button through a clock spring. One of the wires on the horn switch is grounded to the body, so you really just need a single conductor clock spring. I wasn't able to get a good picture, but here's an easy DIY that doesn't look pretty, but it works great and can be hidden. The tab circled in blue is a conductive metal tab that's isolated from ground and rides on a ring (highlighted in yellow) on the backside of the hub that's isolated from the rest of the hub (also ground)... you can provide your switch to ground circuit through this tab and ring. This will avoid needing to run a coiled cord or figuring out how to squeeze a traditional clock spring in there.