drill lots of little holes and then file them to an oval to pass the cable in. you can get braze on outers that will give the correct size hole and give the tube some added strength around the hole. Proper jobs often use a length of stainless tube brazed inside the frame so you just thread the cable through the stainless tube., if you are just rattling round inside the main tube, bend the end of the brake cable inner at an angle so you can rotate it to poke out the hole at the other end of the tube to get the cable through.
drill lots of little holes and then file them to an oval to pass the cable in. you can get braze on outers that will give the correct size hole and give the tube some added strength around the hole. Proper jobs often use a length of stainless tube brazed inside the frame so you just thread the cable through the stainless tube., if you are just rattling round inside the main tube, bend the end of the brake cable inner at an angle so you can rotate it to poke out the hole at the other end of the tube to get the cable through.