I tried the Jagwire kit (which also uses magnets), and it was a nightmare! The magnetic end came off the cable and got lost inside the top tube. I ended up taking off the wheels and shaking the frame around until it spilled out of the opening for the derailleur cables down near the bottom bracket. Pfui. I sent the kit back.
Subsequent to this fiasco, I came up with a solution that works brilliantly: I got a length of vinyl tubing from the hardware store that has an outside diameter small enough to fit through the entrance hole for cable. I fished this through the top tube until it was close to the exit hole. I then attached the end of the tubing to my shop vac (using a lot of duct tape to make a decent seal), then put a some heavy thread near the exit hole. A brief pulse on vac switch was enough to suck the thread right through.
After detaching the vac and pulling off the tubing, I could then attach the thread to the cable. The best way I’ve found was to attach it with super glue. Once through, you can nip off the end, finish the assembly, and Bob’s your uncle!