Based on Glen's suggestion I determined that I could move the egress of the wires 'up' a little and thereby avoid the problem with the stringer.
The orange stuff in the photo is some split tubing I'm using to protect the wiring from the edges of the opening. The lower hole will be covered by the medicine cabinet when it is installed.
If you want others to be happy, practice compassion.
If you want to be happy, practice compassion - The Dalai Lama
1984 310 Limited Motorhome
Courtesy Parking (W/S/E/Wi-Fi) on I-5 in Northern California, 70 miles from Oregon border