Once upon a time I was rehabbing a junker bike to go hiking with: I'd often go on a point-to-point hike, biking back to my car after stashing a beater in the woods before hand. The frame was set up for center pull cantilever brakes with a guide that attached onto the seat bolt clamp. When I got to setting the rear brakes I realized I didn't have the cable stop to insert into the guide (can't remember if I lost it or if it never had it when I got the bike) - and of course none of the others I had in the bits bin fit either. I ended up cutting out the Schrader valve from an old inner tube, removed the core, and stuffed it into the guide. For what it was it worked pretty good - at least for my purposes. I wouldn't advocate for this as a real solution, though

I have a picture of it somewhere but since I hacked this years ago it would probably take me years to now find the pic!