I don't think there is any solution to it?
My frame is not very refined either, just full length cable running into the frame without stops.

attach small zip ties every 10-15 cm, leave the tail attached (do not cut) as you feed the cable through the stops it will push the cable to one side of the frame tube and lessen rattle. Obviously this will only work if your frame opening will allow the head of the zip tie to fit through

If you cant fit the zip ties inside the frame, try putting one at each end of the cable housing that enters the frame after pulling the slack out that is inside the frame then cinch the zip ties tight with a cable puller. I did something similar with the hydraulic brake line that was hitting against my top tube at every bump on my mountain bike. Worth a try if you have extra zip ties.

Good idea, I was using electrical tape to wrap cable housing where it enters the frame, by the end of a long ride that section of cable fell almost completely inside the frame. There is the cable rattling against the entrance hole and then there is cable bumping against the inside of the downtube which I can't control.

If the frame opening was large enough I would add some tiny o-ring instead of zip ties... But still risky to not be able to pull them out when I change cables.