Thanks for the comments, guys. I made another design iteration that I think is an improvement. I'll make a long notch in the top of the stretcher and fill it with a same-sized hardwood panel that has vertical cable slots along the top. It will be held in place with two screws, one at each end, driven vertically down into the stretcher. I can always remove it if it needs to be modified or replaced. Once cables are in place, the lower edge of the removable back panel will keep them captive. It's a simpler, less-fussy design than the split panel, and I like that.
I went ahead and made it longer and with more slots than I currently need - just in case. Since it won't ordinarily be seen, I'm okay with using a piece of cherry for the insert. I'll post more photos once I have it fitted into the stretcher.
