Make an angled post- Geta stock Puch seat post and drill a hole through the side of it that is the same diameter as the new seat post should be. Leave a 3/8" gap from the end for strength.Get a post that matches the new seat clamp diameter and cut it to the length that gives you the amount of setback that you want. Press the new post into the hole and have it welded. Add a bolt to the seat clamp to keep it from sliding off.